Semiconductor device and manufacturing method thereof

ABSTRACT

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

This application is a continuation of copending U.S. application Ser.No. 12/542,068, filed on Aug. 17, 2009 which is a continuation of priorU.S. application Ser. No. 11/524,549 filed on Sep. 21, 2006 (now U.S.Pat. No. 7,674,650 issued Mar. 9, 2010), which are all incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and amanufacturing method thereof and particularly relates to a semiconductordevice using an oxide semiconductor. The present invention also relatesto an electronic appliance equipped with the semiconductor device.

2. Description of the Related Art

Flat panel displays (FPD), typified by liquid crystal displays (LCD) andEL displays, have attracted attention as the display device replacingconventional CRTs. The development of large screen liquid crystaltelevision mounted with an active matrix-driven large scale liquidcrystal panel is particularly an important challenge which liquidcrystal panel makers should focus on. In addition, large screen ELtelevision is also being developed.

In the conventional liquid crystal device or electroluminescence displaydevice (hereinafter referred to as a light emitting display device or anEL display device), a thin film transistor (hereinafter referred to asTFT) is used, which uses crystalline silicon or amorphous silicon as asemiconductor element driving each pixel.

A TFT using a crystalline silicon film has a higher mobility by twodigits or more compared to a TFT using an amorphous silicon film, andhas potential for high speed operation when it is used for a scanningline driver circuit for selecting a pixel of a light emitting displaydevice, a signal line driver circuit for sending video signals to aselected pixel, or the like. However, using crystalline silicon for asemiconductor film complicates manufacturing steps because ofcrystallization of the semiconductor film compared to using amorphoussilicon for the semiconductor film; therefore, there are drawbacks ofyield decrease by that much and increase in cost. Further, a heatingtemperature for the crystallization is 550° C. or higher, and it isdifficult to use a substrate made of a resin with low melting point, aplastic substrate, or the like.

On the other hand, the TFT using amorphous silicon for a semiconductorfilm can be manufactured at low cost, since it is not heated at a hightemperature and a resin substrate or a plastic substrate can be used.However, a mobility of only around 0.2 to 1.0 cm²/V·s at most can beobtained with a TFT of which a channel forming region is formed with asemiconductor film formed of amorphous silicon, and it also has highpower consumption.

A plasma CVD method is commonly used when an amorphous silicon film isformed over a substrate. Film formation by a plasma CVD method requiresheating under high vacuum, and damage to a plastic substrate or anorganic resin film over a substrate is a concern. In addition to theconcern in forming the amorphous silicon film by a plasma CVD method,there is also a concern in forming the film by a sputtering method whichis that a thin insulating film might be formed over a surface of anamorphous silicon film when the amorphous silicon film is exposed toatmospheric air.

As a material to replace a semiconductor made of such silicon, forming aTFT using an oxide semiconductor such as zinc oxide for a channelforming region has been reported in recent years (for example, refer toPatent Document 1: Japanese Patent Laid-Open No. 2000-150900, andNon-Patent document 1: Elvira M. C. Fortunato, et al. Applied PhysicsLetters, Vol. 85, No. 13, P2541 (2004)). Since the oxide semiconductorhas mobility equal to or higher than that of a TFT formed with asemiconductor including amorphous silicon, further characteristicimprovement is demanded.

SUMMARY OF THE INVENTION

In view of the foregoing problems, an object of the present invention isto provide a semiconductor device including a semiconductor element withimproved characteristics and a manufacturing method thereof.

On another front, size increase in substrate has advanced formanufacturing a large-area device by a cheaper process, as in liquidcrystal television. However, with the size increase in substrate, thereis a problem of being easily effected by bending and warping. Also, whena substrate is heated to a high temperature during a heat treatmentstep, a size of the substrate becomes distorted due to warping andshrinking, and there is a problem of a decrease in precision ofalignment in a photolithography step.

Consequently, an object of the present invention is to provide atechnique that makes it possible to manufacture with good yield asemiconductor device over a large substrate, having for example a sidelonger than 1 meter, in a crystallization step of a semiconductorelement used in a semiconductor device.

As mentioned above, an object of the present invention is to provide asemiconductor device including a semiconductor element withcharacteristics that are further improved, which can be manufactured atlower cost and more favorable productivity than before.

In the present invention, a compound semiconductor, more preferably anoxide semiconductor is used as a semiconductor. As the oxidesemiconductor, for example, zinc oxide (ZnO), InGaO₃(ZnO)₅, magnesiumzinc oxide (Mg_(x)Zn_(1-x)O), cadmium zinc oxide (Cd_(x)Zn_(1-x)O),cadmium oxide (CdO), an In—Ga—Zn—O based amorphous oxide semiconductor(a-IGZO), or the like is used. Also, the gist of the present inventionis that by heating a gate electrode that is near the compoundsemiconductor by lamp rapid thermal annealing (LRTA; also simply calledlamp heating), crystallization of the compound semiconductor isselectively promoted, and a TFT using a compound semiconductor havingthe region in which crystallization is promoted at least in a channelregion can be manufactured.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,and an oxide semiconductor film formed over the insulating film. Theoxide semiconductor film includes a first oxide semiconductor region anda second oxide semiconductor region, and the first oxide semiconductorregion that is formed in a position which overlaps with the gateelectrode has higher crystallinity than the second semiconductor region.Note that “crystallinity” expresses a degree of regularity of atomicarrangement inside of crystal, and when manufacturing a TFT using anoxide semiconductor film with favorable crystallinity (also expressed ashaving high crystallinity or with improved crystallinity), an electricalcharacteristic thereof is favorable.

One feature of the present invention is to have a gate electrode and anoxide semiconductor film over a substrate. In a region of the oxidesemiconductor film which overlaps with the gate electrode via aninsulating film, a portion of the region is crystallized.

One feature of the present invention is to have a gate electrode, anoxide semiconductor film, and a conductive film over a substrate. Theconductive film is provided to be in contact with the oxidesemiconductor film, and in a region of the oxide semiconductor filmwhich overlaps with the gate electrode via an insulating film, a portionof the region is crystallized.

One feature of the present invention is to have a gate electrode over asubstrate, an insulating film formed covering the gate electrode, and anoxide semiconductor film formed over the insulating film. The oxidesemiconductor film is crystallized in at least a region which overlapswith the gate electrode. Note that “crystallization” refers togeneration of crystal nuclei from an amorphous state, or growth ofcrystal grains from a state in which crystal nuclei have been generated.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,a conductive film formed over the insulating film, and an oxidesemiconductor film formed over the insulating film and the conductivefilm. The oxide semiconductor film is crystallized in at least a regionwhich overlaps with the gate electrode.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,a conductive film formed over the insulating film, and an oxidesemiconductor film formed over the insulating film and the conductivefilm. The gate electrode has lower reflectivity with respect to a lightsource used for crystallization than the conductive film. Note thatreflectivity comparison is used when the conductive film is a metal filmor the like having a light shielding property.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,a conductive film formed over the insulating film, and an oxidesemiconductor film formed over the insulating film and the conductivefilm. The gate electrode has higher heat absorption rate than theconductive film.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed over the gate electrode, andan oxide semiconductor film formed over the insulating film, and byperforming LRTA on the gate electrode, a portion of the oxidesemiconductor film that overlaps with the gate electrode iscrystallized.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,and an oxide semiconductor film formed over the insulating film. Byperforming LRTA on the gate electrode, a first oxide semiconductorregion and a second oxide semiconductor region are formed inside of theoxide semiconductor film, and the first oxide semiconductor region thatis formed in a position which overlaps with the gate electrode hashigher crystallinity than the second oxide semiconductor region.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed over the gate electrode, aconductive film formed over the insulating film, and an oxidesemiconductor film formed over the insulating film and the conductivefilm. By performing LRTA on the gate electrode, a portion of the oxidesemiconductor film is selectively crystallized.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,an oxide semiconductor film formed over the insulating film, and aconductive film formed over the oxide semiconductor film. By performingLRTA on the gate electrode, a portion of the oxide semiconductor film isselectively crystallized.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,a conductive film formed over the insulating film, and an oxidesemiconductor film formed over the insulating film and the conductivefilm. By performing LRTA on the gate electrode, a first oxidesemiconductor region and a second oxide semiconductor region are formedinside of the oxide semiconductor film. At this time, the first oxidesemiconductor region that is formed in a position which overlaps withthe gate electrode has higher crystallinity than the second oxidesemiconductor region.

One feature of the present invention is to have a gate electrode formedover a substrate, an insulating film formed covering the gate electrode,an oxide semiconductor film formed over the insulating film, and aconductive film formed over the oxide semiconductor film. By lampheating the gate electrode, a first oxide semiconductor region and asecond oxide semiconductor region are formed inside of the oxidesemiconductor film. At this time, the first oxide conductive region thatis formed in a position which overlaps with the gate electrode hashigher crystallinity than the second oxide semiconductor region.

Note that the foregoing conductive film is formed with one element or aplurality of elements selected from Al, Ti, Cu, Au, Ag, Mo, Ni, Ta, Zr,and Co.

Note that it is favorable that the foregoing oxide semiconductor filmincludes at least zinc oxide (ZnO). For example, InGaO₃(ZnO)₅,Mg_(x)Zn_(1-x)O, or Cd_(x)Zn_(1-x)O is given.

Note that the foregoing substrate is any one selected from an organicresin substrate, an inorganic resin substrate, a plastic substrate, anda glass substrate.

Note that the foregoing oxide semiconductor film is formed by asputtering method.

Note that nitrogen may be added to the foregoing oxide semiconductorfilm. When adding nitrogen, nitrogen works as an acceptor impurity whenthe oxide semiconductor film shows an n-type semiconductor property.Consequently, a threshold voltage of a transistor manufactured using anoxide semiconductor film to which nitrogen is added, can be controlled.

One feature of the present invention is to use one of W, TaN, and Cr asa gate electrode, or an alloy including any one thereof.

One feature of the present invention is to perform crystallization of anoxide semiconductor film by irradiation with lamp light of a halogenlamp.

One feature of the present invention is to use light in a wavelengthregion of 800 nm to 2400 nm as lamp light. Also, wavelength in thevisible light region or the infrared light region is used.

One feature of the present invention is a liquid crystal television oran EL television including the foregoing semiconductor device.

Also, in the present invention, a heating treatment may be performed bylaser light irradiation instead of LRTA. For example, laser lightirradiation may be performed using an infrared light laser, a visiblelight laser, an ultraviolet laser, or the like to selectively improvecrystallinity of an oxide semiconductor film. Alternatively, laser lightirradiation may be performed at the same time as performing lamp heatingto selectively improve crystallinity of the oxide semiconductor film.When laser irradiation is used, a continuous wave laser beam (CW laserbeam) or a pulsed laser beam (pulse laser beam) can be used. A laserbeam that can be used here is one or a plurality of that whichoscillates from a gas laser such as an Ar laser, Kr laser, or an excimerlaser; a laser of which a medium is a monocrystalline YAG, YVO₄,forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄ doped with one or more of Nd, Yb,Cr, Ti, Ho, Er, Tm, and Ta, or polycrystalline (ceramic) YAG, Y₂O₃,YVO₄, YAlO₃, or GdVO₄, doped with one or more of Nd, Yb, Cr, Ti, Ho, Er,Tm, and Ta; a glass laser; a ruby laser; an alexandrite laser; aTi:sapphire laser; a copper vapor laser; and a gold vapor laser. Byemitting a laser beam from the second harmonic to the fourth harmonic ofthe fundamental harmonic of such a laser beam, crystallinity can be madeto be favorable. Note that it is preferable to use laser light havinglarger energy than a band gap of the oxide semiconductor film. Forexample, laser light emitted from a KrF, ArF, XeCl, or an XeF excimerlaser oscillator may be used.

In the present invention, a semiconductor device refers to a devicehaving a circuit including a semiconductor element (such as a transistoror a diode), and as the semiconductor device, an integrated circuitincluding a semiconductor element, a display device, a wireless tag, anIC tag, and the like are given. As the display device, a liquid crystaldisplay device, a light emitting device, a DMD (digital micromirrordevice), a PDP (plasma display panel), an FED (field emission display),an electrophoresis display device (electronic paper), and the like aretypically given.

In the present invention, a display device refers to a device using adisplay element, in other words, an image display device. Further, amodule in which a connector, for example an FPC (flexible printedcircuit), a TAB (tape automated bonding) tape, or a TCP (tape carrierpackage), is attached to a display panel; a module provided with aprinted wiring board at an end of the TAB tape or the TCP; and a modulein which an IC (integrated circuit) or a CPU is directly mounted on adisplay element by COG (chip on glass) method are all included as thedisplay device.

In the present invention, it is acceptable as long as crystallization ofan oxide semiconductor film is caused or crystallinity is improved in atleast a channel forming region. Further, the entire channel formingregion is not required to be crystallized, and it is acceptable as longas at least a portion of the channel forming region on a gate electrodeside is crystallized.

Note that as the compound semiconductor, a nitride semiconductor or acarbide semiconductor may be used other than the oxide semiconductor.Further, a semiconductor having a light transmitting property withrespect to visible light can also be used.

In the present invention, crystallinity of a channel forming region ofan oxide semiconductor film is made to be favorable by heating a gateelectrode by LRTA. As a result, the oxide semiconductor film is onlyheated locally; consequently, most of a substrate is not heated, and acrystallization step can be performed as shrinking and bending of thesubstrate are controlled. Consequently, a semiconductor device includinga semiconductor element with improved mobility characteristic can bemanufactured as the step is simplified.

Also, when forming a gate electrode over the substrate, forming aninsulating film functioning as a gate insulating film over the gateelectrode, forming a wiring having higher reflectivity with respect to alight source of LRTA than the gate electrode over the insulating film,and forming a oxide semiconductor film over the wiring, and then LRTA isperformed towards a front surface or a rear surface of a substrate, thewiring is not heated as much as the gate electrode since it has higherreflectivity with respect to the light source of LRTA than the gateelectrode. Therefore, a conductive film having a relatively low meltingpoint such as copper, aluminum, or silver, which has low resistance, canbe used for the wiring. As a result, an inexpensive semiconductor devicecan be provided.

Also, unlike the amorphous silicon film, an insulating film does notform over a surface of the oxide semiconductor film due to oxidationeven if the surface is exposed to an atmosphere containing oxygen.Therefore, even if the oxide semiconductor film is exposed toatmospheric air after formation, there is little change to the film.

Further, when ZnO is used as the oxide semiconductor film, a heattreatment temperature in a crystallization step of the oxidesemiconductor film can be around 350° C. or lower. This is becausecrystallization is sufficiently promoted for ZnO at a heat treatmenttemperature of around 350° C. or lower. As a result, even if a resinsubstrate is used, shrinking of the substrate can be suppressed. Also,lamp heating is performed on the gate electrode using a material havinglower reflectivity with respect to light emitted from a lamp than asource wiring and a drain wiring. Consequently, while crystallinity ofat least a channel forming region of ZnO is improved due to heatconducted from the gate electrode, the source wiring and the drainwiring are not easily heated; therefore, a material having a relativelylow melting point can be used for the source wiring and the drainwiring. For example, since a heat treatment temperature of 350° C. orlower is sufficient when Al is used for the source wiring and the drainwiring, diffusion of Al to a semiconductor layer can be suppressed.

As in the above, since a semiconductor device can be manufactured by alow temperature heat treatment (around 350° C. or lower), it isinexpensive as a process.

Further, since the oxide semiconductor has a light transmittingproperty, by forming the source electrode, the drain electrode, and thelike with a conductive film having a light transmitting property andthen forming a pixel electrode thereover, an aperture ratio of a pixelportion can be improved. When zinc oxide is used as the oxidesemiconductor, since resource of zinc oxide is more abundant than thatof indium tin oxide (ITO) and since zinc oxide has lower resistance, amore inexpensive semiconductor device can be obtained by using zincoxide instead of ITO as the pixel electrode. When silicon is used for asemiconductor film, in order to prevent the channel forming region frombeing irradiated with light, it is necessary to provide a lightshielding film so as to overlap the channel forming region. As a result,a decrease in aperture ratio of a pixel portion is unavoidable. On theother hand, when zinc oxide is used for an oxide semiconductor film,since resource of zinc oxide is relatively abundant and since zinc oxidehas a light transmitting property, by forming each of a sourceelectrode, a drain electrode, and a pixel electrode using a transparentconductive material including indium tin oxide (ITO), ITSO made ofindium tin oxide and silicon oxide, organic indium, organic tin, zincoxide, titanium nitride, or the like each having a light transmittingproperty, a large scale display with high aperture ratio in atransmissive type display panel can be obtained. Also, light from abacklight can be effectively used to save power. For example, bysticking a display panel over a window of a building or a windshield ofan automobile, a train, an airplane, or the like, a head-up display inwhich an image or text information is directly displayed can berealized.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and B are each a cross-sectional view describing amanufacturing step of a semiconductor device relating to the presentinvention;

FIG. 2 is a diagram describing temperature dependency of crystallizationof an oxide semiconductor film of the present invention;

FIGS. 3A to 3C are each a cross-sectional view describing amanufacturing step of a semiconductor device relating to the presentinvention;

FIGS. 4A to 4H are each a cross-sectional view describing amanufacturing step of a semiconductor device relating to the presentinvention;

FIGS. 5A to 5C are each a cross-sectional view describing amanufacturing step of a semiconductor device relating to the presentinvention;

FIGS. 6A to 6F are each a cross-sectional view describing amanufacturing step of a semiconductor device relating to the presentinvention;

FIG. 7 is a cross sectional-view of a semiconductor device relating tothe present invention;

FIGS. 8A to 8F are each a diagram showing a mode of a light emittingelement relating to the present invention;

FIGS. 9A to 9F are each a diagram describing a pixel circuit of adisplay panel relating to the present invention and an operationconfiguration thereof;

FIGS. 10A to 10C are each a diagram describing mounting of a drivercircuit relating to the present invention;

FIG. 11 is a diagram describing a display module relating to the presentinvention;

FIGS. 12A to 12F are each a diagram describing one example of anelectronic appliance;

FIGS. 13A and 13B are each a cross-sectional view of a semiconductordevice relating to the present invention;

FIGS. 14A and 14B are each a circuit diagram and a cross-sectional viewof a pixel in a semiconductor device of the present invention;

FIG. 15 is a cross-sectional view of a semiconductor device relating tothe present invention;

FIG. 16 is a diagram showing one mode of an element substrate in asemiconductor device of the present invention;

FIGS. 17A and 17B are each a diagram showing one mode of an elementsubstrate in a semiconductor device of the present invention;

FIGS. 18A and 18B are each a block diagram showing a structure of asemiconductor device of the present invention;

FIGS. 19A and 19B are each a diagram showing a structure of an LRTAdevice relating to the present invention;

FIG. 20 describes one example of an electronic appliance relating to thepresent invention;

FIG. 21 describes one example of an electronic appliance relating to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

Embodiment modes of the present invention will hereinafter be describedwith reference to drawings. However, the invention is not limited to thefollowing description, and it is easily understood by those skilled inthe art that the modes and details can be changed in various wayswithout departing from the spirit and scope of the invention. Therefore,the invention is not interpreted limited to the following description ofembodiment modes.

Embodiment Mode 1

In this embodiment mode, a manufacturing step of a TFT using a channelforming as a region of an oxide semiconductor film in whichcrystallinity is improved by LRTA, is described with reference to FIGS.1A and 1B.

First, a base film 102 is formed over a substrate 101. For the substrate101, glass, or plastic (synthetic resin) such as polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone(PES), acrylic, or polyimide can be used.

As the base film 102, a single layer of an insulating film such as asilicon oxide film, a silicon nitride film, a silicon oxynitride film(SiO_(x)N_(y)) (x>y), or a silicon nitride oxide film (SiN_(x)O_(y))(x>y), or stacked layers thereof are used. The base film 102 may beformed by a sputtering method or a CVD method. Note that the base film102 is not always required to be provided, but it is preferable to formin the present invention. By forming the base film 102, conduction ofheat generated from an electrode or a wiring formed over the base film102 to the substrate 101 can be suppressed. As the base film 102, asilicon nitride oxide film with a thickness of 10 to 400 nm can be used,for example.

Subsequently, a gate electrode 103 is formed over the base film 102. Thegate electrode 103 with a thickness of 100 to 200 nm may be formed by asputtering method. The gate electrode 103 can be formed using an elementselected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum(Mo), chromium (Cr), niobium (Nb), or the like, or an alloy material ora compound material mainly containing such an element. Further, the gateelectrode 103 can also be formed with a semiconductor material typifiedby polycrystalline silicon doped with an impurity element such asphosphorous.

Subsequently, a gate insulating film 104 with a thickness of about 50 to500 nm is formed to cover the gate electrode 103. The gate insulatingfilm 104 may be formed to have a single layer structure of a filmcontaining an oxide of silicon or a nitride of silicon, or as a stackedlayer structure thereof, by a sputtering method or a variety of CVDmethods such as a plasma CVD method. Specifically, a film containingsilicon oxide (SiO_(x)), a film containing silicon oxynitride(SiO_(x)N_(y)), or a film containing silicon nitride oxide(SiN_(x)O_(y)) is formed as a single layer structure, or these films areappropriately stacked to form the gate insulating film 104. Also, thegate insulating film may be formed by performing high density plasmatreatment on the gate electrode 103 under an atmosphere containingoxygen, nitrogen, or both oxygen and nitrogen and oxidizing or nitridinga surface of the gate electrode 103. The gate insulating film formed bya high density plasma treatment has excellent uniformity in its filmthickness, film quality, and the like and the film can be formed to bedense. As the atmosphere containing oxygen, a mixed gas of a noble gas,oxygen (O₂), and nitrogen dioxide (NO₂), or dinitrogen monoxide (N₂O);or a mixed gas of a noble gas, hydrogen (H₂), and oxygen (O₂), nitrogendioxide (NO₂), or dinitrogen monoxide (N₂O), can be used. Also, as theatmosphere containing nitrogen, a mixed gas of a noble gas and nitrogen(N₂) or ammonia (NH₃); or a mixed gas of a noble gas, hydrogen (H₂), andnitrogen (N₂) or ammonia (NH₃), can be used. By an oxygen radical (mayalso include an OH radical) or a nitrogen radical (may also include a NHradical) generated by the high density plasma treatment, the surface ofthe gate electrode 103 can be oxidized or nitrided.

When the gate insulating film 104 is formed by performing the highdensity plasma treatment, the insulating film with a thickness of 1 to20 nm, preferably 5 to 10 nm, is formed covering the gate electrode 103.Since a reaction in this case is a solid-phase reaction, interface statedensity of between the gate insulating film 104 and the gate electrode103 can be made to be extremely low. Further, since the gate electrode103 is oxidized or nitrided directly, a thickness of the gate insulatingfilm 104 to be formed can be uniform. Consequently, by solid-phaseoxidation of the surface of the electrode by the high density plasmatreatment shown here, an insulating film with favorable uniformity andlow interface state density can be formed. Here, an oxide of an elementselected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum(Mo), chromium (Cr), niobium (Nb), or the like; or an oxide of an alloymaterial or a compound material mainly containing the element functionsas the gate insulating film 104.

Note that for the gate insulating film 104, just an insulating filmformed by the high density plasma treatment may be used, or at least oneof an insulating film of silicon oxide, silicon nitride containingoxygen, silicon oxide containing nitrogen, and the like may be stackedin addition thereover by a CVD method utilizing plasma or heat reaction.Either way, transistors each of which a gate insulating film ispartially or entirely an insulating film formed by high density plasmacan be made to have little variations in characteristic.

The gate insulating film 104 may use the following which have favorablecompatibility with the oxide semiconductor film: alumina (Al₂O₃),aluminum nitride (AlN), titanium oxide (TiO₂), zirconia (ZrO₂), lithiumoxide (Li₂O), potassium oxide (K₂O), sodium oxide (Na₂O), indium oxide(In₂O₃), yttrium oxide (Y₂O₃), or calcium zirconate (CaZrO₃); or amaterial including at least two thereof. The gate insulating film 104may be formed as a single layer or as stacked layers of two or morelayers.

Subsequently, a wiring 105 with a thickness of 50 to 200 nm is formedover the gate insulating film 104. As a wiring material, silver (Ag),aluminum (Al), gold (Au), copper (Cu), an alloy thereof or the like isused. It is acceptable as long as the wiring material has higherreflectivity than that of the material used for the gate electrode 103,and the wiring material is appropriately combined and used taking intoconsideration the gate electrode 103. Note that the wiring may be formedto have a stacked layer structure. For example, aluminum and titaniummay be stacked over the substrate in this order to form a wiring with astacked layer structure. Titanium is effective in making an electricalcontact property between the oxide semiconductor film and aluminumfavorable. Titanium also takes on a role of suppressing diffusion ofaluminum to the oxide semiconductor film. Also, the wiring may be formedwith a transparent conductive film, such as for example indium tin oxide(ITO), indium tin oxide containing silicon oxide (ITSO), indium zincoxide (IZO), indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO),zinc oxide added with aluminum (AlZnO), zinc oxide added with gallium(GaZnO), or zinc oxide. Note that for the wiring 105, it is favorable touse a material having higher reflectivity or higher transmissivity (orlower heat absorption rate) with respect to lamp light than that of thegate electrode 103.

Next, an oxide semiconductor film 106 is formed over the gate insulatingfilm 104 and the wiring 105. For the oxide semiconductor film 106, zincoxide (ZnO) in an amorphous state, a polycrystalline state, or amicrocrystalline state in which both amorphous and polycrystallinestates exist, added with one type or a plurality of types of impurityelements selected from the following can be used: a Group 1 element (forexample, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), orcesium (Cs)), a Group 13 element (for example, boron (B), gallium (Ga),indium (In), or thallium (TI)), a Group 14 element (for example, carbon(C), silicon (Si), germanium (Ge), tin (Sn), or lead (Pb)), a Group 15element (for example, nitrogen (N), phosphorus (P), arsenic (As),antimony (Sb), or bismuth (Bi)), a Group 17 element (for example,fluorine (F), chlorine (Cl), bromine (Br), or iodine (I)), or the like.Alternatively, zinc oxide (ZnO) in an amorphous state, a polycrystallinestate, or a microcrystalline state in which both amorphous andpolycrystalline states exist, which is not added with any impurityelement can also be used. Further, any of the following can also beused: InGaO₃(ZnO)₅, magnesium zinc oxide (Mg_(x)Zn_(1-x)O), cadmium zincoxide (Cd_(x)Zn_(1-x)O), cadmium oxide (CdO), or an In—Ga—Zn—O basedamorphous oxide semiconductor (a-IGZO). The oxide semiconductor film 106is formed by forming a film with a thickness of 25 to 200 nm (preferably30 to 150 nm) by a sputtering method under conditions of a pressure of0.4 Pa and a flow rate of Ar(argon):O₂=50:5 (sccm) to form into adesired pattern, then subsequently etching the film using fluorinatedacid diluted to 0.05%. Compared to a semiconductor film using anamorphous silicon film, the oxide semiconductor film 106 does not needto be formed under high vacuum since there is no concern for oxidation,and is inexpensive as a process. Note that since an oxide semiconductorfilm containing zinc oxide is resistant against plasma, a plasma CVD(also called PCVD or PECVD) method may be used to form the film. AmongCVD methods, the plasma CVD method in particular uses a simple device,and has favorable productivity.

Subsequently, LRTA is performed towards a rear surface of the substrate101 (FIG. 1A). LRTA is performed at 250° C. to 570° C. (preferably 300°C. to 400° C., more preferably 300° C. to 350° C.) for 1 minute to 1hour, preferably 10 minutes to 30 minutes. LRTA is performed withradiation from one type or a plurality types of lamps selected from ahalogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp,a high pressure sodium lamp, and a high pressure mercury lamp. Since aheat treatment in a short amount of time is possible with an LRTAmethod, a material with a relatively low melting point can be used ifreflectivity or transmissivity of the wiring 105 is higher than that ofthe gate electrode 103. For the LRTA method, light of a wavelength inthe infrared light region, the visible light region, the ultravioletlight region, or the like can be used. Note that instead of LRTA, aheating treatment may be performed by laser light irradiation, and forexample, laser light of an infrared light laser, a visible light laser,an ultraviolet laser, or the like may be used. Alternatively, LRTA andlaser light irradiation may be combined to selectively improvecrystallinity of the oxide semiconductor film. When laser irradiation isused, a continuous wave laser beam (CW laser beam) or a pulsed laserbeam can be used. A laser beam that can be used here is one or aplurality of that which oscillates from a gas laser such as an Ar laser,Kr laser, or an excimer laser, a laser of which a medium is amonocrystalline YAG, YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄ dopedwith one or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta, orpolycrystalline (ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄, doped withone or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta; a glass laser; a rubylaser; an alexandrite laser; a Ti:sapphire laser; a copper vapor laser;and a gold vapor laser. By emitting a laser beam from the secondharmonic to the fourth harmonic of the fundamental harmonic of such alaser beam, crystallinity can be made to be favorable. Note that it ispreferable to use laser light having larger energy than a band gap ofthe oxide semiconductor film. For example, laser light emitted from aKrF, ArF, WeCl, or an XeF excimer laser oscillator may be used.

At this time, since the gate electrode 103 is formed with a materialthat has lower reflectivity with respect to lamp light and that whichabsorbs more heat than that of the wiring 105, the gate electrode 103 isheated to a higher temperature than the wiring 105. For this reason, theoxide semiconductor film 106 in a periphery of the gate electrode 103 isheated; consequently, a second oxide semiconductor region 108 and afirst oxide semiconductor region 107 with more favorable crystallinitythan the second oxide semiconductor region 108 are formed (see FIG. 1B).Here, the gate electrode 103 is irradiated with lamp light so as to beheated to around 300° C., and by that heat, the oxide semiconductor film106 is crystallized to improve crystallinity. At this time, since amaterial with higher reflectivity or transmissivity with respect to lamplight than that of the gate electrode 103 is used, a temperature of thewiring 105 is 300° C. or less even if the oxide semiconductor film 106is crystallized.

Here, a heat treatment temperature dependency of a crystallinity of ZnOused as the oxide semiconductor film is shown in FIG. 2. FIG. 2 shows aresult of measuring an X-ray intensity of a (002) surface in each of thefollowing cases: a case where a deposition gas with a flow rate ratio ofAr:O₂=50:5 (sccm) is sprayed (as-deposited); and cases when thedeposition gas is sprayed and then heated for 1 hour at each temperatureof 200° C., 300° C., and 350° C. As heat treatment temperature rises, anintensity peak of the (002) surface is greater. Consequently, at leastup to 350° C., crystallinity of ZnO increases as the heat treatmenttemperature rises. Since mobility increases in general ascrystallization progresses, it is desirable to perform the heattreatment at around 350° C. Note that if there is no problem such asshrinking of the substrate, a heat treatment in which ZnO is heated toaround 400° C. may be performed.

On the other hand in FIG. 1A, in a region in which the gate electrode103 and the wiring 105 are not formed, in other words, in a region inwhich the substrate 101, the base film 102, the gate insulating film104, and the oxide semiconductor film 106 are stacked, lamp light istransmitted through compared to a region in which the wiring 105 and thegate electrode 103 are formed; consequently, heat is not easily absorbedand a heating temperature is lower than that of the wiring 105.Consequently, since a large region of the substrate 101 is 350° C. orlower, shrinking does not occur easily. Note that the larger the regionin which the gate electrode 103 is not formed, shrinking of thesubstrate 101 is suppressed.

Next, a semiconductor device is manufactured by forming an interlayerinsulating film, a source electrode, a drain electrode, a pixelelectrode, a light emitting element, and the like over the oxidesemiconductor film 106.

In the present invention, when ZnO is used as a semiconductor,crystallinity of a ZnO layer is improved with a heat treatmenttemperature of about 300° C.; therefore, compared to when a crystallinesilicon film is used as a semiconductor film, the heat treatmenttemperature is suppressed. Also, since an oxide semiconductor filmhaving a high light transmitting property is used and a gate electrodeis selectively heated by LRTA, most of a substrate is not heated andshrinking of the substrate can be suppressed. Further, since a materialused for a wiring has higher reflectivity with respect to lamp lightthan that of the gate electrode, crystallinity of the oxidesemiconductor film can be improved even if a temperature to which thewiring is heated is suppressed to around 350° C. Therefore, an Al wiringwhich has a low melting point can be used. Also, formation of aninsulating film due to diffusion of oxygen in the oxide semiconductorfilm to the Al can be prevented. Since the Al wiring is inexpensive andhas low resistance, a semiconductor device with favorable performancecan be manufactured at low cost and with favorable productivity.

Embodiment Mode 2

In this embodiment mode, a structure that is different from that inEmbodiment Mode 1 is described with reference to FIGS. 3A to 3C. Notethat steps of forming a base film 302, a gate electrode 303, and a gateinsulating film 304 over a substrate 301 corresponds to the steps offorming the base film 102, the gate electrode 103, and the gateinsulating film 104 over the substrate 101 of Embodiment Mode 1,respectively; therefore, refer to Embodiment Mode 1 for the steps.

A first oxide semiconductor film 305 is formed over the gate insulatingfilm 304. For the oxide semiconductor film 305, zinc oxide (ZnO) in anamorphous state, a polycrystalline state, or a microcrystalline state inwhich both amorphous and polycrystalline states exist, added with onetype or a plurality of types of impurity elements selected from Group 1elements, Group 13 elements, Group 14 elements, Group 15 elements, andGroup 17 elements can be used. Alternatively, zinc oxide (ZnO) in anamorphous state, a polycrystalline state, or a microcrystalline state inwhich both amorphous and polycrystalline states exist, which is notadded with any impurity element can also be used. Further, any of thefollowing can also be used: InGaO₃(ZnO)₅, magnesium zinc oxide(Mg_(x)Zn_(1-x)O), cadmium zinc oxide (Cd_(x)Zn_(1-x)O), cadmium oxide(CdO), or an In—Ga—Zn—O based amorphous oxide semiconductor (a-IGZO).Here, zinc oxide is formed to a thickness of 50 to 200 nm (preferably100 to 150 nm) as the first oxide semiconductor film 305 by a sputteringmethod.

Subsequently, LRTA is performed towards a substrate surface to makecrystallinity favorable (FIG. 3A). LRTA may be performed at 250° C. to570° C. (preferably at 300° C. to 400° C., and more preferably at 300°C. to 350° C.) for 1 minute to 1 hour, preferably 10 minutes to 30minutes. LRTA is performed with radiation from one type or a pluralityof types of lamps selected from a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, and ahigh pressure mercury lamp. In this embodiment mode, lamp heating isperformed on the gate electrode 303 for 30 minutes in an oxygenatmosphere so that the gate electrode becomes about 300° C., in order toimprove crystallinity of a region of the first oxide semiconductor film305 which overlaps the gate electrode 303 with the gate insulating filmtherebetween. Since the first oxide semiconductor film 305 has a lighttransmitting property, the gate electrode 303 is heated with priority,and crystallinity of the first oxide semiconductor film 305 increasesfrom a periphery of the gate electrode 303 towards the outside. Then, asshown in FIG. 3B, a second oxide semiconductor film including a secondoxide semiconductor region 309 and a first oxide semiconductor region308 with more favorable crystallinity than the second oxidesemiconductor region 309 are formed. Note that in FIG. 3A, although lampheating is performed towards a front surface side of the substrate 301,LRTA may be performed towards a rear surface of the substrate. Since theoxide semiconductor film 305 has a light transmitting property, mostregion of the substrate is not easily heated even if LRTA is performed.Consequently, deformation such as shrinking of the substrate can besuppressed even if a resin with a low melting point or the like is usedfor the substrate. Note that crystallinity of a surface of the oxidesemiconductor film and a periphery thereof may be improved directly byperforming lamp heating towards the substrate surface with LRTA withincreased output. Also, for the oxide semiconductor film overlappingwith the gate electrode, a surface of the oxide semiconductor film on agate insulating layer 304 side and a periphery thereof may becrystallized with priority when performing lamp heating towards thesubstrate surface, by adjusting wavelength of lamp light, reflectivityof the gate electrode, and film thickness of the oxide semiconductorfilm, so that lamp light reflecting off of the gate electrode isabsorbed by the surface of the oxide semiconductor film on the gateinsulating layer 304 side and the periphery thereof. Further, when aglass substrate is used for the substrate, lamp light used is of thevisible light region to the infrared light region. Since light in thesewavelength regions is not easily absorbed by the glass substrate,heating of the glass substrate can be suppressed to a minimum. Note thatlamp heating may be performed a plurality of times. By performing lampheating a plurality of times, heating time can be gained at the sametime as suppressing a rise in a temperature of the substrate.

Note that instead of LRTA, crystallinity of the oxide semiconductor filmmay be selectively improved by laser light irradiation, ultravioletirradiation, or by a combination thereof. When laser irradiation isused, a continuous wave laser beam (CW laser beam) or a pulsed laserbeam (pulse laser beam) can be used. A laser beam that can be used hereis one or a plurality of that which oscillates from a gas laser such asan Ar laser, Kr laser, or an excimer laser; a laser of which a medium isa monocrystalline YAG, YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄ dopedwith one or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta, orpolycrystalline (ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄, doped withone or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta; a glass laser; a rubylaser; an alexandrite laser; a Ti:sapphire laser; a copper vapor laser;and a gold vapor laser. By emitting a laser beam from the secondharmonic to the fourth harmonic of the fundamental harmonic of such alaser beam, crystallinity can be made to be favorable. Note that it ispreferable to use laser light having larger energy than a band gap ofthe oxide semiconductor film. For example, laser light emitted from aKrF, ArF, XeCl, or an XeF excimer laser oscillator may be used.

Subsequently, over the first oxide semiconductor region 308 and thesecond oxide semiconductor region 309, Ti and Al are deposited by asputtering method to form a Ti layer and an Al layer. After that, awiring 306 and a wiring 307 are formed as a source wiring and a drainwiring by performing dry etching on the Ti layer and the Al layer usingphotolithography and Cl₂ gas (FIG. 3C). The wirings 306 and 307 are eachformed to have a thickness of 10 to 200 nm by using an accelerationvoltage of 1.5 kw, a pressure of 0.4 Pa, and Ar (flow rate of 30 sccm).Note that although the wirings 306 and 307 are formed as stacked layers,if materials used for the wiring 306 and 307 have favorablecompatibility with the oxide semiconductor film 305, the wirings 306 and307 may be formed in a single layer. As the material for each of thewirings 306 and 307, a metal such as aluminum (Al), tungsten (W),molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium(Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), platinum(Pt), titanium (Ti), or neodymium (Nd), or an alloy or a metal nitridethereof can be appropriately used. Alternatively, a material having alight transmitting property such as indium tin oxide (ITO), indium zincoxide (IZO), indium tin oxide containing silicon oxide (ITSO), indiumoxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), zinc oxide added withaluminum (AlZnO), zinc oxide added with gallium (GaZnO), or the like canbe appropriately used.

Subsequently, a semiconductor device is manufactured by forming aninterlayer insulating film, a wiring, a pixel electrode, a lightemitting element and the like over the oxide semiconductor film 305, thewiring 306 and the wiring 307.

In this embodiment mode, a wiring is formed after performing LRTA on theoxide semiconductor film 305 to improve crystallinity. Therefore, amaterial having lower reflectivity with respect to lamp light than thatof the gate electrode 303 may be used for the wiring 306, and thematerial for the wiring is not limited to those mentioned in EmbodimentMode 1 as long as it has favorable compatibility with the oxidesemiconductor film 305.

Note that after the oxide semiconductor film 305 is formed, heating byLRTA may be performed before or after processing the oxide semiconductorfilm 305 into a desirable shape.

In the present invention, when zinc oxide is used for a semiconductorfilm, since crystallinity of the semiconductor film improves at a heattreatment temperature of around 300° C., heat treatment temperature canbe suppressed and a crystallization step can be performed at low costcompared to when a crystalline silicon film is used as the semiconductorfilm. Further, since a gate electrode is selectively heated by LRTAusing an oxide semiconductor film having a high light transmittingproperty, most of a substrate is not heated and shrinking of thesubstrate can be suppressed.

Embodiment Mode 3

An embodiment mode of the present invention is described with referenceto FIGS. 4A to 5C. This embodiment mode is an example of a semiconductordevice including a channel protective thin film transistor.

As a substrate 400, a glass substrate including barium borosilicateglass, alumino borosilicate glass, or the like; a silicon substrate; aplastic substrate having heat resistance; or a resin substrate is used.As the plastic substrate or the resin substrate, polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone(PES), acrylic, polyimide, or the like can be used. Also, a surface ofthe substrate 400 may be polished by a CMP method so that the surface isplanarized. Note that an insulating layer may be formed over thesubstrate 400. The insulating layer is formed to have a single layerstructure or a stacked layer structure using at least one of an oxidematerial including silicon and a nitride material including silicon, bya known method such as a CVD method, a plasma CVD method, a sputteringmethod, or a spin coating method. This insulating layer is notnecessarily formed, but it has effects of blocking contaminants and thelike from the substrate 400, as well as suppressing conduction of heatto the substrate.

A conductive film 401 is formed over the substrate 400. The conductivefilm 401 is processed into a desired shape and becomes a gate electrode.The conductive film 401 is preferably formed by a method such as aprinting method, an electrolytic plating method, or an evaporationmethod, using a material having a low reflectivity with respect to awavelength of a light source used for LRTA heating (a material whicheasily absorbs heat, in other words, that which is easily heated). Byusing the material having a low reflectivity, a subsequent heating stepbecomes possible. As the conductive film 401, a metal such as tungsten(W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V),niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni),platinum (Pt), titanium (Ti), or neodymium (Nd), or an alloy or a metalnitride thereof can be appropriately used. Further, the conductive film401 may have a stacked layer structure of a plurality of these layers.Typically, a tantalum nitride film may be stacked over a substratesurface, and then a tungsten film may be stacked thereover. Further,silicon added with an impurity element imparting one conductivity typemay also be used. For example, an n-type silicon film of an amorphoussilicon film including an impurity element imparting n-type such asphosphorus (P) can be used. The conductive film 401 is formed to have athickness of 10 nm to 200 nm.

In this embodiment mode, the conductive film 401 is formed to have athickness of 150 nm by a sputtering method using tungsten (W).

A mask made of a resist is formed over the conductive film 401 using aphotolithography step, and the conductive film 401 is processed into adesired shape using the mask to form a gate electrode 402 (see FIG. 4B).

Subsequently, a gate insulating film 403 a and a gate insulating film403 b are formed over the gate electrode 402 so as to have a stackedlayer structure of two layers. The stacked insulating films may beformed consecutively in the same chamber without breaking a vacuum andunder the same temperature, by changing reaction gases. By forming theinsulating films consecutively without breaking the vacuum,contamination of an interface between the stacked films can beprevented.

For the gate insulating film 403 a and the gate insulating film 403 b,silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), silicon nitride oxide (SiN_(x)O_(y)) (x>y), or thelike can be appropriately used. Also, instead of the gate insulatingfilm 403 a, the gate electrode 402 may be oxidized to form an oxidefilm. Note that to prevent diffusion of impurities and the like from thesubstrate, the gate insulating film 403 a is preferably formed usingsilicon nitride (SiN_(x)), silicon nitride oxide (SiN_(x)O_(y)) (x>y),or the like. Further, the gate insulating film 403 b is desirably formedusing silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y),or the like. Note that in order to form a dense insulating film withlittle gate leak current at a low deposition temperature, it isfavorable to include a noble gas element such as argon in a reaction gasso that the noble gas element is incorporated in the insulating film tobe formed. In this embodiment mode, the gate insulating film 403 a isformed using a silicon nitride film with a thickness of 50 nm to 140 nmthat is formed with SiH₄ and NH₃ as reaction gases, and the gateinsulating film 403 b is formed using a silicon oxide film with athickness of 100 nm that is formed with SiH₄ and N₂O as reaction gases,and stacked thereover. Note that it is preferable that the gateinsulating film 403 a and the gate insulating film 403 b each have athickness of 50 nm to 100 nm.

Alternatively, the gate insulating film 403 b may be formed usingalumina (Al₂O₃) or aluminum nitride (AlN) each having favorablecompatibility with an oxide semiconductor film to be subsequentlyformed. In this case, by using silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, or the like having a high insulatingproperty for the gate insulating film 403 a, and using alumina oraluminum nitride having a favorable interface property with respect tothe oxide semiconductor film for the gate insulating film 403 b, a highreliability gate insulating film can be formed. The gate insulating filmmay have three layers, and the third layer may be a gate insulating filmusing alumina or aluminum nitride.

Subsequently, an oxide semiconductor film 404 is formed over the gateinsulating film 403 b. The oxide semiconductor film 404 may be formed tohave a thickness of 100 nm by a sputtering method under the followingconditions: a flow rate of Ar:O₂=50:5 (sccm), and a pressure of 0.4 Pa.

For the oxide semiconductor film 404, ZnO in an amorphous state, apolycrystalline state, or a microcrystalline state in which bothamorphous and polycrystalline states exist, added with one type or aplurality of types of impurity elements selected from Group 1 elements,Group 13 elements, Group 14 elements, Group 15 elements, and Group 17elements can be used. Alternatively, ZnO in an amorphous state, apolycrystalline state, or a microcrystalline state in which bothamorphous and polycrystalline states exist which is not added with anyimpurity element can also be used. Further, any of the following canalso be used: InGaO₃(ZnO)₅, magnesium zinc oxide (Mg_(x)Zn_(1-x)O),cadmium zinc oxide (Cd_(x)Zn_(1-x)O), cadmium oxide (CdO), or anIn—Ga—Zn—O based amorphous oxide semiconductor (a-IGZO).

Note that when ZnO is used for the oxide semiconductor film 404, it isfavorable that ZnO is added (doped) with nitrogen. ZnO normally shows ann-type semiconductor property. By adding nitrogen, since nitrogen worksas an acceptor with respect to ZnO, a threshold voltage can besuppressed as a result.

Subsequently, heating of the oxide semiconductor film 404 is performedtowards a front surface or a rear surface of the substrate 400 by anLRTA method (see FIG. 4D). LRTA is performed with radiation from one ora plurality of lamps selected from a halogen lamp, a metal halide lamp,a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, and ahigh pressure mercury lamp. LRTA is performed at 250° C. to 570° C.(preferably 300° C. to 400° C., more preferably 300° C. to 350° C.) for1 minute to 1 hour, preferably 10 minutes to 30 minutes. In thisembodiment mode, lamp heating is performed with a halogen lamp as alight source, and in an oxygen atmosphere at 300° C. for 30 minutes.

By performing LRTA, the gate electrode 402 is selectively heated in ashort amount of time, and a first oxide semiconductor region withimproved crystallinity is formed by heat thereof in a region 434 formedin a periphery of the gate electrode 402, which is indicated by a dottedline. On the other hand, a region 424 that is not the region 434indicated by the dotted line is barely heated since there is littleabsorption of lamp light, and a second oxide semiconductor region havinga different crystallinity from that of the first oxide semiconductorregion (see FIG. 4E). Consequently, since only a region in which thegate electrode 402 is formed is selectively heated and the other regionis not heated, shrinking and bending of the substrate 400 can besuppressed. Note that crystallinity in a periphery of the surface of theoxide semiconductor film may be improved directly by performing lampheating towards the substrate surface with LRTA with increased output.Also, for the oxide semiconductor film overlapping with the gateelectrode, a surface of the oxide semiconductor film on a gateinsulating layer 403 b side and a periphery thereof may be crystallizedwith priority when performing lamp heating towards the substratesurface, by adjusting wavelength of lamp light, reflectivity of the gateelectrode, and film thickness of the oxide semiconductor film, so thatlamp light reflecting off of the gate electrode is absorbed by thesurface of the oxide semiconductor film on the gate insulating layer 403b side and the periphery thereof. Further, when a glass substrate isused for the substrate, lamp light used is of the visible light regionto the infrared light region. Since light in these wavelength regions isnot easily absorbed by the glass substrate, heating of the glasssubstrate can be suppressed to a minimum. Note that lamp heating may beperformed a plurality of times. By performing lamp heating a pluralityof times, heating time can be gained at the same time as suppressing arise in a temperature of the substrate.

Note that instead of LRTA, crystallinity of the oxide semiconductor filmmay be selectively improved by laser light irradiation, ultravioletirradiation, or by a combination thereof. When laser irradiation isused, a continuous wave laser beam (CW laser beam) or a pulsed laserbeam (pulse laser beam) can be used. A laser beam that can be used hereis one or a plurality of that which oscillates from a gas laser such asan Ar laser, Kr laser, or an excimer laser; a laser of which a medium isa monocrystalline YAG, YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄ dopedwith one or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta, orpolycrystalline (ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄, doped withone or more of Nd, Yb, Cr, Ti, Ho, Er, Tm, and Ta; a glass laser; a rubylaser; an alexandrite laser; a Ti:sapphire laser; a copper vapor laser;and a gold vapor laser. By emitting a laser beam from the secondharmonic to the fourth harmonic of the fundamental harmonic of such alaser beam, crystallinity can be made to be favorable. Note that it ispreferable to use laser light having larger energy than a band gap ofthe oxide semiconductor film. For example, laser light emitted from aKrF, ArF, XeCl, or an XeF excimer laser oscillator may be used.

Subsequently, a protective film 405 is formed over the oxidesemiconductor film 404, and a resist 406 is formed over the protectivefilm 405 (see FIG. 4F). By a photolithography step using the resist 406as a mask, the protective film 405 is processed into a desired shape toform a channel protective film 407. As the channel protective film,silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), silicon nitride oxide (SiN_(x)O_(y)) (x>y), or thelike can be appropriately used. By forming the channel protective film407, a semiconductor layer of a channel portion can be prevented frombeing etched when a source electrode and a drain electrode are formed.In this embodiment mode, silicon nitride is formed as the protectivefilm 405, and then the channel protective film 407 is formed (see FIG.4G).

Subsequently, a mask 408 is manufactured with a resist (FIG. 4H), andetching is performed on the oxide semiconductor film 404 to process intoa desired shape by a photolithography step using the mask 408, to forman oxide semiconductor film 409 (also called island-shaped oxidesemiconductor film) (FIG. 5A). Note that diluted fluorinated acid isused for the etching. Subsequently, a first conductive film 411 and asecond conductive film 412 are formed over the oxide semiconductor film409, and a mask 413 is formed by a photolithography step with a resist(FIG. 5B). The first conductive film 411 and the second conductive film412 are processed into desired shapes using the mask 413, and firstconductive films 414 a and 414 b, and second conductive films 415 a and415 b each functioning as a source electrode or a drain electrode areformed (FIG. 5C).

As the mask, a commercially available resist material including aphotosensitizing agent may be used. For example, a typical positive typeresist, such a novolac resin or a naphthoquinone diazide compound whichis a photosensitizing agent; or a negative type resist, such as a baseresin, diphenylsilanediol, or an acid generator may be used. In usingany of the materials, surface tension and viscosity thereof isappropriately adjusted by adjusting a concentration of a solvent, or byadding a surfactant or the like. Also, when a conductive materialincluding a photosensitive substance having photosensitivity is used forthe conductive films, the conductive films can be processed into desiredshapes by being subjected to direct laser light irradiation, exposure,and removal with an etchant, without forming a mask from resist. In thiscase, there is an advantage that a step is simplified since a mask isnot required to be formed.

As the conductive material including a photosensitive substance, amaterial including a metal such as Ag, Au, Cu, Ni, Al, or Pt, or analloy thereof; an organic high molecular compound resin; a photopolymerization initiator; a photopolymerization monomer; and aphotosensitive resin made of a solvent or the like, may be used. As theorganic high molecular resin, a novolac resin, an acrylic copolymer, amethacrylic copolymer, a cellulose derivative, a cyclic rubber resin, orthe like is used.

Note that before forming the first conductive film 411, one more layerof a conductive film made of for example zinc oxide added with aluminum(AlZnO) or zinc oxide added with gallium (GaZnO) may be provided as ann-type semiconductor, over the oxide semiconductor film 404. By formingthe conductive film made of AlZnO or GaZnO, compatibility between thefirst conductive film 411 and the oxide semiconductor film 409 becomesfavorable, and a contact resistance between the oxide semiconductor film409 and a source electrode and a drain electrode can be reduced.Alternatively, for example, a stacked layer structure of forming Ti overGaZnO, or forming GaZnO over Ti may be provided.

As the first conductive films 414 a and 414 b and the second conductivefilms 415 a and 415 b, a metal such as aluminum (Al), tungsten (W),molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium(Nb), tantalum (Ta), copper (Cu), chromium (Cr), cobalt (Co), nickel(Ni), platinum (Pt), titanium (Ti), or neodymium (Nd), or an alloy or ametal nitride thereof can be appropriately used. For example, thefollowing combinations of the first conductive films 414 a, 414 b andthe second conductive films 415 a, 415 b can be considered: Ti and Al;Ta and W; TaN and Al; and TaN and Cu; as the first conductive films andthe second conductive films, respectively. Also, a combination of athird conductive film using Ti in addition to the first conductive filmsusing Ti and the second conductive films using Al can be considered.Further, an AgPdCu alloy may be used for one of a first layer and asecond layer. Furthermore, a structure may be a three-layer stackedlayer structure of sequentially stacking W, an alloy of Al and Si(Al—Si), and TiN. Note that tungsten nitride, an alloy film of Al and Ti(Al—Ti), and Ti may be used instead of W, the alloy of Al and Si(Al—Si), and TiN, respectively. In order to improve heat resistance, anelement such as titanium, silicon, scandium, neodymium, or copper may beadded to aluminum at 0.5 to 5 atomic %.

As a conductive material to form the first conductive film 411 and thesecond conductive film 412, a material having a light transmittingproperty such as indium tin oxide (ITO), indium zinc oxide (IZO), indiumtin oxide containing silicon oxide (ITSO), indium oxide (In₂O₃), tinoxide (SnO₂), or zinc oxide (ZnO), or an appropriate combination thereofmay be used.

In this embodiment mode, the first conductive film 411 and the secondconductive film 412 are formed after LRTA is performed on the oxidesemiconductor film 305 and crystallinity thereof is improved. Therefore,a material having lower reflectivity with respect to lamp light thanthat of the gate electrode 402 may be used for the first conductive film411 and the second conductive film 412, and a conductive material for awiring or an electrode is not limited to those mentioned in EmbodimentMode 1 as long as it has favorable compatibility with the oxidesemiconductor film 305.

In this embodiment mode, either plasma etching (dry etching) or wetetching may be employed for an etching process; however, plasma etchingis suitable for treating a substrate with a large area. As an etchinggas, a fluorinated acid based gas such as CF₄, NF₃, SF₆, or CHF₃; achlorine based gas typified by Cl₂, BCl₃, SiCl₄, CCl₄, or the like; oran O₂ gas may be used, to which an inert gas such as He or Ar may beappropriately added. Also, by applying an etching process usingatmospheric pressure discharge, electric discharge machining is possiblelocally, and a mask layer is not required to be formed on the an entiresurface of the substrate.

Before applying the resist in the photolithography step of thisembodiment mode, an insulating film with a thickness of about several nmmay be formed over a surface of the oxide semiconductor film. By thisstep, the oxide semiconductor film and the resist coming into directcontact with each other can be avoided, and entering of impuritiesincluded in the resist into the oxide semiconductor film can beprevented.

By the above steps, a bottom gate type (also called reverse staggeredtype) thin film transistor in which a semiconductor layer of a channelportion is not etched can be manufactured. Note that although a bottomgate type TFT is manufactured in this embodiment mode, a top gate typeTFT may be formed as long as crystallinity of at least a channel formingregion of an oxide semiconductor film can be improved by heating a gateelectrode that is formed over an oxide semiconductor film formed over asubstrate, with a gate insulating film therebetween.

This embodiment mode can be appropriately combined with Embodiment Modes1 and 2.

Embodiment Mode 4

An embodiment mode of the present invention is described with referenceto FIGS. 6A to 6F. This embodiment mode is an example of a semiconductordevice according to Embodiment Mode 3 having a channel etch type thinfilm transistor. Therefore, repeated description of the same portions orthe portions having similar functions is omitted.

A gate electrode 602 is formed over a substrate 600, and a gateinsulating film 603 a and a gate insulating film 603 b are formedcovering the gate electrode 602 (FIG. 6A). An oxide semiconductor film620 is formed over the gate insulating film 603 b, and LRTA is performedtowards a substrate surface to form an oxide semiconductor film 620including a first oxide semiconductor region 604 with improvedcrystallinity in a region indicated by a dotted line, and a second oxidesemiconductor region 605 in which crystallization is not as progressedas the first oxide semiconductor region 604 (see FIG. 6B). A mask 608 isprovided over the oxide semiconductor film (FIG. 6C), and the oxidesemiconductor film is processed into a desired shape by aphotolithography step to form an oxide semiconductor film 609 (FIG. 6D).

Next, a first conductive film 611 and a second conductive film 612 areformed. Then, a mask 613 made of a resist is formed (see FIG. 6E). Inthis embodiment mode, conductive films containing titanium and aluminumare formed by a sputtering method as each of the first conductive film611 and the second conductive film 612.

Subsequently, the first conductive film 611 and the second conductivefilm 612 are processed into a desired shape using the mask 613 by aphotolithography step, and first conductive films 615 a and 615 b, andsecond conductive films 616 a and 616 b each functioning as a sourceelectrode or a drain electrode are formed (FIG. 6F).

By the above steps, a thin film transistor in which a semiconductorlayer of a part of a channel portion is etched can be manufactured.

Note that in this embodiment mode, one more layer of a conductive filmmade of for example zinc oxide added with aluminum (AlZnO) or zinc oxideadded with gallium (GaZnO) may be provided as an n-type oxidesemiconductor, between the oxide semiconductor film and the firstconductive film 611. Alternatively, for example, a stacked layerstructure of forming Ti over GaZnO, or forming GaZnO over Ti may beprovided. By forming an n-type oxide semiconductor film, connectionbetween the first conductive film 611 that becomes a source electrode ora drain electrode and the oxide semiconductor film can be made to befavorable, and a contact resistance can be reduced.

This embodiment mode can be appropriately combined with Embodiment Modes1 to 3.

Embodiment Mode 5

In this embodiment mode, a light emitting device which a bottom gatetype thin film transistor formed in Embodiment Mode 3 or Embodiment Mode4 is connected to a pixel electrode is described with reference to FIG.7. Note that a thin film transistor of this embodiment mode is achannel-etched type.

FIG. 7 shows a cross-sectional view of a TFT used in a driver circuitand a cross-sectional view of a TFT used in a pixel portion. A referencenumeral 701 denotes a cross-sectional view of a TFT used in a drivercircuit, a reference numeral 702 denotes a cross-sectional view of a TFTused in a pixel portion, and a reference numeral 703 denotes across-sectional view of a light emitting element provided with a currentby the TFT 702. The TFTs 701 and 702 are bottom gate types.

The TFT 701 of the driver circuit includes a gate electrode 710 formedover a substrate 700; a gate insulating film 711 covering the gateelectrode 710; and an oxide semiconductor film 712 containing zinc oxidewhich overlaps with the gate electrode 710 with the gate insulating film711 interposed therebetween. Further, the TFT 701 includes firstconductive films 713 each functioning as a source electrode or a drainelectrode, and second conductive films 714 each functioning as a sourceelectrode or a drain electrode. Note that the first conductive films 713and the second conductive films 714 also function as wiring.

In FIG. 7, the gate insulating layer 711 is formed of two layers ofinsulating films; however, the present invention is not limited to thisstructure. The gate insulating film 711 may be formed with a singlelayer of an insulating film or three or more layers of insulating films.

The second conductive films 714 are formed with aluminum or an alloycontaining aluminum. Also, the second conductive films 714 that are apair face each other with a channel forming region of the oxidesemiconductor film 712 in therebetween.

Further, the first conductive films 713 are formed with titanium. Thefirst conductive films 713 are not required to be provided; however,electrical contact property of the second conductive film 711 with theoxide semiconductor film 712 becomes favorable. Also, the firstconductive films 713 have a function as barrier layers for preventingdiffusion of oxygen in the oxide semiconductor film 712 to the secondconductive films 714. As a result, reliability of a TFT can be improved.Note that an oxide semiconductor film is known to show an n-type withoutperforming anything thereto. Therefore, the first oxide semiconductorfilm in which a channel is formed may have its conductivity typecontrolled in advance so as to be close to an i-type (also called as anintrinsic-type that is defined as a conductivity type having an equalnumber of negative and positive charges) as much as possible, by addingan impurity imparting p-type conductivity.

The TFT 702 of the pixel portion includes a gate electrode 720 formedover the substrate 700, the gate insulating film 711 covering the gateelectrode 720, and an oxide semiconductor film 722 which overlaps withthe gate electrode 720 with the gate insulating film 711 interposedtherebetween. Further, the TFT 702 includes first conductive films 723each functioning as a source electrode or a drain electrode, and secondconductive films 724 each functioning as a source electrode or a drainelectrode.

The second conductive films 724 are formed with aluminum or an alloycontaining aluminum. Also, the second conductive films 724 that are apair face each other with a region in which a channel of the oxidesemiconductor film 722 is formed in between.

Further, the first conductive films 723 are formed with titanium. Thefirst conductive films 723 are not required to be provided; however,electrical contact property of the second conductive film 724 with theoxide semiconductor film 722 becomes favorable. Also, the firstconductive films 723 have a function as barrier layers for preventingdiffusion of oxygen in the oxide semiconductor film 722 to the secondconductive films 724. As a result, reliability of a TFT can be improved.Note that an oxide semiconductor film is known to show an n-type withoutperforming anything thereto. Therefore, the first oxide semiconductorfilm in which a channel is formed may have its conductivity typecontrolled in advance so as to be close to an i-type as much aspossible, by adding an impurity imparting p-type conductivity.

Also, a first passivation film 740 and a second passivation film 741each formed of an insulating film are formed covering the TFTs 701 and702. The first passivation film 740 and the second passivation film 741can be formed by a thin film formation method such as a plasma CVDmethod or a sputtering method, using an insulating material such assilicon nitride, silicon oxide, silicon nitride oxide, siliconoxynitride, aluminum oxynitride, aluminum oxide, diamond-like carbon(DLC), nitrogen-containing carbon (CN), or the like. The passivationfilms covering the TFTs 701 and 702 is not limited to two layers, and asingle layer or three or more layers may be provided. For example, thefirst passivation film 740 and the second passivation film 741 can beformed of silicon nitride and silicon oxide, respectively. By forming apassivation film of silicon nitride or silicon nitride oxide, enteringof impurities from outside into a semiconductor element can beprevented, and degradation of the TFTs 701 and 702 due to an effect ofmoisture or the like can be prevented. In this embodiment mode, thefirst passivation film 740 and the second passivation film 741 areconsecutively formed in the same chamber by performing gas switching.

Next, one of the second conductive films 724 is connected to a pixelelectrode of a light emitting element 703.

Subsequently, an insulating layer 729 (also called partition, or bank)is selectively formed. The insulating layer 729 is formed so as to havean opening portion over the pixel electrode 730 and so as to cover thesecond passivation film 741. In this embodiment mode, the insulatinglayer 729 is formed covering an entire surface, and then etched using amask of a resist or the like to form into a desired shape.

The insulating layer 729 can be formed with an inorganic insulatingmaterial such as silicon oxide, silicon nitride, silicon oxynitride,aluminum oxide, aluminum nitride, or aluminum oxynitride; an inorganicsiloxane based insulating material having an Si—O—Si bond amongcompounds made of silicon, oxygen, and hydrogen, using a siloxane basedmaterial as a starting material; or an organic siloxane based materialin which hydrogen bonded with silicon is substituted with an organicgroup such as methyl or phenyl. Also, the insulating layer 729 may beformed using a photosensitive or a non-photosensitive material such asan acrylic resin, or a polyimide resin. The insulating layer 729preferably has a form of which a curvature radius changes continuously,so that coatability of an electric field light emitting layer 731 and anopposing electrode 732 are improved.

Subsequently, the electric field light emitting layer 731 is formed overthe pixel electrode 730 so as to be in contact therewith. As theelectric field light emitting layer 731, materials showing lightemission of red (R), green (G), and blue (B), respectively, are eachselectively formed by an evaporation method or the like using anevaporation mask. The materials showing light emission of red (R), green(G), and blue (B), respectively, are preferable since they can be formedby a droplet discharging method in a similar manner to a color filter(such as a low molecular compound or a high molecular compound), and inthis case, RGB can be applied separately without using a mask. Note thatother than a three-color combination of RGB, the combination may be withfour colors by adding emerald green. Also, vermilion may be added.Further, a pixel including an EL element that emits white light may becombined.

The opposing electrode 732 is formed so as to be in contact with theelectric field light emitting layer 731. Note that although the lightemitting element 703 includes an anode and a cathode, one is used as apixel electrode, and the other is used as an opposing electrode. In thisway, a light emitting device having a display function using a lightemitting element is completed.

In the present invention, since a channel forming region of an oxidesemiconductor film includes at least a crystallized region, a TFT havinghigher mobility than that of a TFT using an amorphous silicon film canbe obtained. Also, since a crystallization step is performed at a lowertemperature than that of a TFT using a crystalline silicon film, it isinexpensive as a process.

This embodiment mode can be appropriately combined with Embodiment Modes1 to 4.

Embodiment Mode 6

In this embodiment mode, a liquid crystal display device in which asemiconductor element made of the bottom gate type thin film transistorto which the present invention is applied and a pixel electrode areconnected, is described with reference to FIGS. 13A to 18B. Note thatEmbodiment Mode 5 can be referred to regarding the formation up to thesecond passivation film 741; therefore, the same reference numerals areused as those of FIG. 7, and descriptions thereof are omitted.

As in FIG. 13A, after the second passivation film 741 is formed, aninsulating layer 1329 is formed so as to cover the second passivationfilm 741.

Subsequently, wirings 1371, 1372, 1373, and 1374 connected to the secondconductive films 714 and 724, respectively, are formed via contactholes. Then, the second conductive films 724 are electrically connectedto a pixel electrode 1330 of a liquid crystal element 1303 via thewiring 1374. For the pixel electrode 1330, in a case of manufacturing atransmissive type liquid crystal display panel, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, or the like can be used. Of course, indium tin oxide (ITO),indium zinc oxide (IZO), indium tin oxide added with silicon oxide(ITSO), or the like can be used. Also, in a case of manufacturing areflective type display panel, as a metal thin film having a reflectiveproperty, a conductive film made of titanium, tungsten, nickel, gold,platinum, silver, aluminum, magnesium, calcium, lithium, an alloythereof, or the like can be used. The pixel electrode 1330 can be formedby an evaporation method, a sputtering method, a CVD method, a printingmethod, a droplet discharging method, or the like.

Further, an orientation film 1331 is formed over the pixel electrode1330 so as to be in contact therewith. Under a second substrate 1340facing the first substrate 700 with the pixel electrode 1330therebetween, an opposing electrode 1341 and an orientation film 1342are stacked in this order. Also, a liquid crystal 1343 is providedbetween the pixel electrode 1330 and the orientation film 1331 andbetween the opposing electrode 1341 and the orientation film 1342, and aportion where the pixel electrode 1330, the liquid crystal 1343, and theopposing electrode 1341 overlap each other corresponds to a liquidcrystal element 1303. Note that the pixel electrode 1330 may be formedto extend over the TFT 702, as shown in FIG. 13B. Since an oxidesemiconductor film has a light transmitting property with respect tovisible light, when a transparent conductive film including indium tinoxide (ITO), ITSO made of indium tin oxide and silicon oxide, organicindium, organic tin, zinc oxide, titanium nitride, or the like eachhaving a light transmitting property, an aperture ratio of a pixelportion can be improved.

Note that a distance (cell gap) between the pixel electrode 1330 and theopposing electrode 1341 is controlled by a spacer 1361. Although in FIG.13A, the spacer 1361 is formed by processing an insulating film providedon a first substrate 700 side into a desired shape, spacers preparedseparately may be dispersed over the orientation film 1331 to controlthe cell gap. A reference numeral 1362 denotes a sealant, and by thesealant 1362, the liquid crystal 1343 is sealed between the firstsubstrate 700 and the second substrate 1340.

Further, on a surface of the first substrate 700 that is not the surfaceover which the TFT 701 and the TFT 702 are formed, a polarizing plate1350 is provided. Also, on a surface of the second substrate 1340 thatis not the surface over which the opposing electrode 1341 is formed, apolarizing plate 1351 is provided. Note that the number of orientationfilms and polarizing plates, and positions thereof in a liquid crystaldisplay device of the present invention are not limited to those shownin a structure of FIG. 13A.

In the present invention, since at least crystallization of a channelforming region of an oxide semiconductor film is improved, a TFT havinghigher mobility than that of a TFT using an amorphous silicon film canbe obtained. Also, since a crystallization step is performed at a lowertemperature than that of a TFT using a crystalline silicon film, it isinexpensive as a process. Further, since crystallinity of the oxidesemiconductor film is selectively increased by lamp heating, the time ittakes for crystallization can be shortened compared to when the entireoxide semiconductor film is crystallized. Therefore, yield can beincreased. Also, since crystallization is performed selectively and in ashort amount of time, shrinking of a substrate does not occur easily,and a substrate having a relatively low melting point such as a resinsubstrate can be used. Consequently, a TFT can be manufactured at lowcost.

Also, since the channel forming region does not absorb visible light,unnecessary photocarriers are not generated. Therefore, a TFT withexcellent light resistance can be formed.

Subsequently, a different structure of a pixel included in a liquidcrystal display device of the present invention is described. FIG. 14Ashows one mode of a circuit diagram of the pixel, and FIG. 14B shows onemode of a cross-sectional structure of the pixel corresponding to FIG.14A.

In FIGS. 14A and 14B, a reference numeral 1501 denotes a switching TFTfor controlling input of video signal to the pixel, and a referencenumeral 1502 denotes a liquid crystal element. Specifically, potentialof a video signal that is input to the pixel via the switching TFT 1501is supplied to a pixel electrode of the liquid crystal element 1502.Note that a reference numeral 1503 denotes a capacitor element forretaining voltage between the pixel electrode of the liquid crystalelement 1502 and an opposing electrode when the switching TFT 1501 isturned off.

Specifically, gate electrodes of the switching TFT 1501 are connected toa scanning line G and one of a source region and a drain region isconnected to a signal line S, and the other is connected to a pixelelectrode 1504 of the liquid crystal element 1502. One of two electrodesincluded in the capacitor element 1503 is connected to the pixelelectrode 1504 of the liquid crystal element 1502, and the other issupplied with a constant potential, desirably a potential that is of thesame level as that of the opposing electrode.

Note that in FIGS. 14A and 14B, a structure is that of a multi-gatestructure in which the switching TFT 1501 is serially connected and aplurality of TFTs to which gate electrodes 1510 are connected share anoxide semiconductor film 1512. By having the multi-gate structure, anoff current of the switching TFT 1501 can be reduced. Specifically,although in FIGS. 14A and 14B, a structure of the switching TFT 1501 isthat of two TFTs being serially connected to each other, it may be amulti-gate structure in which three or more TFTs are serially connectedto each other, and in which the gate electrodes are also connected.Further, the switching TFT is not required to have a multi-gatestructure, and it may be a TFT of a regular single-gate structure inwhich one gate electrode and one channel forming region are provided.

Next, a mode of a TFT included in a liquid crystal display device of thepresent invention that is different from that of FIGS. 13A to 14B isdescribed. FIG. 15 shows a cross-sectional view of a TFT used in adriver circuit, and a cross-sectional view of a TFT used in a pixelportion. A reference numeral 2301 denotes the cross-sectional view of aTFT used in a driver circuit, a reference numeral 2302 denotes thecross-sectional view of a TFT used in a pixel portion, and a referencenumeral 2303 denotes a cross-sectional view of a liquid crystal element.

The TFT 2301 of the driver circuit includes a gate electrode 2310 formedover a substrate 2300, a gate insulating film 2311 covering the gateelectrode 2310, and an oxide semiconductor film 2312 including acrystallized region in at least a channel forming region, that overlapswith the gate electrode 2310 with the gate insulating film 2311therebetween. Also, the TFT 2302 of the pixel portion includes a gateelectrode 2320 formed over the substrate 2300, the gate insulating film2311 covering the gate electrode 2320, and an oxide semiconductor film2322 including a crystallized region in at least a channel formingregion, that overlaps with the gate electrode 2320 with the gateinsulating film 2311 therebetween. Further, channel protective films2390 and 2391 formed of insulating films are formed so as to cover thechannel forming regions of the oxide semiconductor films 2312 and 2322.The channel protective films 2390 and 2391 are provided to prevent thechannel forming regions of the oxide semiconductor films 2312 and 2322from getting etched during manufacturing steps of the TFT 2301 and 2302.Furthermore, the TFT 2301 includes first conductive films 2313 eachfunctioning as a source electrode or a drain electrode and, secondconductive films 2314 each functioning as a source electrode or a drainelectrode; and the TFT 2302 includes first conductive films 2323 eachfunctioning as a source electrode or a drain electrode and secondconductive films 2324 each functioning as a source electrode or a drainelectrode. Note that the first conductive films 2313 and 2323, and thesecond conductive films 2314 and 2324 function as wirings layers.

In FIG. 15, the gate insulating layer 2311 is formed of two layers ofinsulating films; however the present invention is not limited to thisstructure. The gate insulating film 2311 may be formed with a singlelayer of an insulating film or three or more layers of insulating films.

The second conductive films 2314 and 2324 are formed with aluminum or analloy containing aluminum. Also, the second conductive films 2314 thatare a pair and the second conductive films 2324 that are a pair faceeach other with a region in which a channel of the oxide semiconductorfilm 2322 is formed in between.

Further, the first conductive films 2313 and 2323 are formed withtitanium. The first conductive films 2313 and 2323 are not required tobe provided; however, electrical contact property of the secondconductive films 2314 and 2324 with the oxide semiconductor films 2312and 2322 becomes favorable. Also, the first conductive films 2313 and2323 have a function as barrier layers for preventing diffusion ofoxygen in the oxide semiconductor films 2312 and 2322 to the secondconductive films 2314 and 2324. As a result, reliability of a TFT can beimproved. Note that the oxide semiconductor films 2312 and 2322 areknown to show an n-type without performing anything thereto. Therefore,the first oxide semiconductor films in which channels are formed mayhave their conductivity type controlled in advance so as to be close toan i-type as much as possible, by adding an impurity imparting p-typeconductivity.

Also, a first passivation film 2380 and a second passivation film 2381each formed of an insulating film are formed covering the TFTs 2301 and2302. The first passivation film 2380 and the second passivation film2381 can be formed by a thin film formation method such as a plasma CVDmethod or a sputtering method, using an insulating material such assilicon nitride, silicon oxide, silicon nitride oxide, siliconoxynitride, aluminum oxynitride, aluminum oxide, diamond-like carbon(DLC), nitrogen-containing carbon (CN), etc. The passivation filmscovering the TFTs 2301 and 2302 are not limited to two layers, and asingle layer or three or more layers may be provided. For example, thefirst passivation film 2380 and the second passivation film 2381 can beformed with silicon nitride and silicon oxide, respectively. By forminga passivation film with silicon nitride or silicon nitride oxide,entering of impurities from outside into a semiconductor element can beprevented, and degradation of the TFTs 2301 and 2302 due to an effect ofmoisture or the like can be prevented. In this embodiment mode, thefirst passivation film 2380 and the second passivation film 2381 areconsecutively formed in the same chamber by performing gas switching.

Subsequently, an insulating layer 2329 is formed covering the secondpassivation films 2381. Next, wirings 2371, 2372, 2373, and 2374connected to the second conductive films 2314 and 2324, respectively,are formed via contact holes. Then, the conductive film 2324 iselectrically connected to a pixel electrode 2330 of the liquid crystalelement 2302 via the wiring 2374.

An orientation film 2331 is formed over the pixel electrode 2330 so asto be in contact there with. Under a second substrate 2340 facing thefirst substrate 2300 with the pixel electrode 2330 therebetween, anopposing electrode 2341 and an orientation film 2342 are stacked in thisorder. Also, a liquid crystal 2343 is provided between the pixelelectrode 2330 and the orientation film 2331 and between the opposingelectrode 2341 and the orientation film 2342, and a portion where thepixel electrode 2330, the liquid crystal 2343, and the opposingelectrode 2341 overlap each other corresponds to a liquid crystalelement 2303. Note that the pixel electrode may be formed to extend overthe TFT. When a transparent conductive film including indium tin oxide(ITO), ITSO made of indium tin oxide and silicon oxide, organic indium,organic tin, zinc oxide, titanium nitride, or the like each having alight transmitting property is used for the first conductive film andthe second conducive film, an aperture ratio of a pixel portion can beimproved.

Note that a distance (cell gap) between the pixel electrode 2330 and theopposing electrode 2341 is controlled by a spacer 2361. Although in FIG.15, the spacer 2361 is formed by processing an insulating film into adesired shape, spacers prepared separately may be dispersed over theorientation film 2331 to control the cell gap. A reference numeral 2362denotes a sealant, and by the sealant 2362, the liquid crystal 2343 issealed between the first substrate 2300 and the second substrate 2340.

Further, on a surface of the first substrate 2300 that is not thesurface over which the TFT 2301 and the TFT 2302 are formed, apolarizing plate is provided (not shown). Also, on a surface of thesecond substrate 2340 that is not the surface over which the opposingelectrode 2341 is formed, a polarizing plate is provided (not shown).Note that the number of orientation films and polarizing plates, andpositions thereof in a liquid crystal display device of the presentinvention are not limited to those shown in a structure of FIG. 15.

Next, a structure of an element substrate used in a liquid crystaldisplay device of the present invention is shown.

FIG. 16 shows a mode of an element substrate in which a pixel portion6012 formed over a first substrate 6011 is connected to a separatelyformed signal line driver circuit 6013. The pixel portion 6012 and thescanning line driver circuit 6014 are each formed using a TFT includingan oxide semiconductor film including a crystallized region in at leasta channel forming region. By forming the signal line driver circuit witha transistor by which higher mobility can be obtained compared to thatof a TFT using an amorphous silicon film, operation of the signal linedriver circuit which demands higher driving frequency than that of thescanning line driver circuit can be stabilized. Note that the signalline driver circuit 6013 may be a transistor using a monocrystallinesilicon semiconductor, a TFT using a polycrystalline semiconductor, or atransistor using SOI. The pixel portion 6012, the signal line drivercircuit 6013, and the scanning line driver circuit 6014 are eachsupplied with potential of a power source, various signals, and the likevia an FPC 6015.

Note that the signal driver circuit and the scanning line driver circuitmay both be formed over the same substrate as that of the pixel portion.

Also, when the driver circuit is separately formed, a substrate overwhich the driver circuit is formed is not always required to be stuckover a substrate over which the pixel portion is formed, and may bestuck for example over the FPC. FIG. 17A shows a mode of an elementsubstrate in which a pixel portion 6022 formed over a first substrate6021 is connected to a separately formed signal line driver circuit6023. The pixel portion 6022 and the scanning line driver circuit 6024are each formed with a TFT using an oxide semiconductor film including acrystallized region in at least a channel forming region. The signalline driver circuit 6023 is connected to the pixel portion 6022 via anFPC 6025. The pixel portion 6022, the signal line driver circuit 6023,and the scanning line driver circuit 6024 are each supplied withpotential of a power source, a variety of signals, and the like via theFPC 6025.

Also, just a portion of the signal line driver circuit or just a portionof the scanning line driver circuit may be formed over the samesubstrate as that of the pixel portion using the TFT including an oxidesemiconductor film including a crystallized region in at least a channelforming region, and the rest may be formed separately to be electricallyconnected to the pixel portion. FIG. 17B shows a mode of an elementsubstrate where an analog switch 6033 a included in the signal drivercircuit is formed over a first substrate 6031, which is the samesubstrate as that over which a pixel portion 6032 and a scanning linedriver circuit 6034 are formed, and forming a shift register 6033 bincluded in the signal line driver circuit over a different substrateseparately and then sticking it over the substrate 6031. The pixelportion 6032 and the scanning line driver circuit 6034 are each formedusing the TFT including an oxide semiconductor film including acrystallized region in at least a channel forming region. The shiftregister 6033 b included in the signal line driver circuit is connectedto the pixel portion 6032 via an FPC 6035. The pixel portion 6032, theanalog switch 6033 a and shift register 6033 b included in the signalline drive circuit, and the scanning line driver circuit 6034 are eachsupplied with potential of a power source, a variety of signals, and thelike via the FPC 6035.

As shown in FIG. 16 to FIG. 17B, in a liquid crystal display device ofthe present invention, an entire driver circuit or a portion thereof canbe formed over the same substrate as that of a pixel portion, using theTFT including an oxide semiconductor film including a crystallizedregion in at least a channel forming region.

Note that a connection method of a separately formed substrate is notparticularly limited, and a COG (chip on glass) method, a wire bondingmethod, a TAB (tape automated bonding) method or the like can be used.Further, a connection position is not limited to the position shown inFIGS. 18A and 18B, as long as electrical connection is possible. Also, acontroller, a CPU, a memory, or the like may be formed separately andconnected.

Note that a signal line driver circuit used in the present invention isnot limited to a mode including only a shift register and an analogswitch. In addition to the shift register and the analog switch, anothercircuit such as a buffer, a level shifter, or a source follower may beincluded. Also, the shift register and the analog switch is not alwaysrequired to be provided, and for example a different circuit such as adecoder circuit by which selection of signal line is possible may beused instead of the shift register, and a latch or the like may be usedinstead of the analog switch.

FIG. 18A shows a block diagram of a liquid crystal display device towhich the present invention is applied. The liquid crystal displaydevice shown in FIG. 18A includes a pixel portion 801 including aplurality of pixels and provided with a liquid crystal element; ascanning line driver circuit 802 that selects each pixel; and a signalline driver circuit 803 that controls video signal input to a selectedpixel.

In FIG. 18A, the signal line driver circuit 803 includes a shiftregister 804 and an analog switch 805. To the shift register 804, aclock signal (CLK) and a start pulse signal (SP) are input. When theclock signal (CLK) and the start pulse signal (SP) are input, timingsignals are generated in the shift register 804, and the timing signalsare input to the analog switch 805.

Also, the analog switch 805 is provided with video signals. The analogswitch 805 samples the video signals according to the timing signals anddistributes the video signals to a signal line of a latter stage.

Next, a structure of the scanning line driver circuit 802 is described.The scanning line driver circuit 802 includes a shift register and abuffer 807. Also, a level shifter may be included in some cases. In thescanning line driver circuit 802, by inputting the clock signal (CLK)and the start pulse signal (SP), a selection signal is generated. Thegenerated selection signal is buffer amplified in the buffer 807, andthen supplied to a corresponding scanning line. To the scanning line,gates of transistors in pixels of one line are connected. Further, sincethe transistors in the pixels of one line have to be turned on at thesame time, a buffer to which a large current can be fed is used for thebuffer 807.

In a full color liquid crystal display device, when a video signalcorresponding to each of R (red), G (green), and B (blue) are sampled insequence and each are supplied to a corresponding signal line, thenumber of terminals for connecting the shift register 804 and the analogswitch 805 corresponds to about ⅓ of the number of terminals forconnecting the analog switch 805 and the pixel portion 801.Consequently, by forming the analog switch 805 and the pixel portion 801over the same substrate, terminals used for connecting separately formedsubstrates are not required as in a case of forming the analog switch805 and the pixel portion over different substrates, and occurrenceprobability of poor connection can be suppressed, and yield can beincreased.

FIG. 18B shows a block diagram of a liquid crystal display device towhich the present invention is applied that is different from that ofFIG. 18A. In FIG. 18B, a pixel portion 811 is shown, and a signal linedriver circuit 813 includes a shift register 814, a latch A 815, a latchB 816, and a D/A converter circuit (hereinafter referred to as a DAC817). A scanning line driver circuit 812 is to have the same structureas that of the scanning line driver circuit 802 in FIG. 18A.

To the shift register 814, the clock signal (CLK) and the start pulsesignal (SP) are input. When the clock signal (CLK) and the start pulsesignal (SP) are input, timing signals are generated in the shiftregister 814 to be input in sequence to the latch A 815 of a firststage. When the timing signals are input to the latch A 815, videosignals are written to the latch A 815 in synchronism with the timingsignals and retained. Note that in FIG. 18B, although it is assumed thatthe video signals are written to the latch A 815 in sequence, thepresent invention is not limited to this structure. A so called divisiondrive in which a plurality of stages of the latch A 815 are divided intoseveral groups, and video signals are input in parallel for each group.Note that the number of the groups at this time is called a divisionnumber. For example, when the latches are divided into groups in each offour stages, this is called division driving with four divisions.

The time it takes for a video signal writing to a latch of the latch A815 in all of the stages to complete is called a line period. Inpractice, a line period sometimes includes the line period to which ahorizontal retrace line period is added.

When one line period is completed, latch signals are supplied to thelatch B 816 of a second stage, and video signals retained in the latch A815 are written all at once in synchronism with the latch signals, andretained. To the latch A 815 which have sent the video signals to thelatch B 816, subsequent video signals are written in sequence insynchronism with timings signals from the shift register 814. In thissecond round of the one line period, video signals written and retainedin the latch B 816 are input to DAC 817.

The DAC 817 converts input video signals from digital to analog, andsupplies the signals to a corresponding signal line.

Note that the configurations shown in FIGS. 18A and 18B are modes of aliquid crystal display device relating to this embodiment mode, andconfigurations of a signal line driver circuit and a scanning linedriver circuit are not limited thereto.

Note that FIGS. 16 to 18B is not used just for a liquid crystal displaydevice relating to this embodiment mode, and can be used for a lightemitting device or other display devices.

Note that this embodiment mode can be appropriately combined withEmbodiment Modes 1 to 4.

Embodiment 1

This embodiment describes a mode of a light emitting element used in thelight emitting device described in Embodiment Mode 5, with reference toFIGS. 8A to 8F.

FIG. 8A shows an example of forming a first pixel electrode 11 by usinga conductive film having a light transmitting property and a high workfunction and forming a second pixel electrode 17 by using a conductivefilm having a low work function. The first pixel electrode 11 is formedof an oxide conductive material having a light transmitting property,typically, an oxide conductive material containing a silicon oxide at aconcentration of 1 to 15 atomic %. A layer containing a light emittingsubstance 16 composed of a hole injecting or transporting layer 41, alight emitting layer 42, an electron transporting or injecting layer 43is formed over the first pixel electrode 11. The second pixel electrode17 is composed of a first electrode layer 33 containing an alkali metalor an alkali earth metal such as LiF or MgAg and a second electrodelayer 34 formed of a metal material such as aluminum. The pixel havingsuch the structure can emit light from the first pixel electrode 11 sideas indicated by arrow in the drawing.

FIG. 8B shows an example of forming a first pixel electrode 11 by usinga conductive film having a high work function and forming a second pixelelectrode 17 by using a conductive film having a light transmittingproperty and a low work function. The first pixel electrode 11 iscomposed of a first electrode layer 35 formed of a metal such asaluminum or titanium, or the metal and a metal material containingnitrogen at a concentration of a stoichiometric composition ratio orless, and a second electrode layer 32 formed of an oxide conductivematerial containing silicon oxide at a concentration of 1 to 15 atomic%. A layer containing a light emitting substance 16 composed of a holeinjecting or transporting layer 41, a light emitting layer 42, anelectron transporting or injecting layer 43 is formed over the firstpixel electrode 11. The second pixel electrode 17 is composed of a thirdelectrode layer 33 containing an elemental substance of an alkali metalor an alkali earth metal such as LiF or CaF or a compound or an alloythereof, and a fourth electrode layer 34 formed of a metal material suchas aluminum. By forming each the third electrode layer 33 and the fourthelectrode layer 34 to have a thickness of 100 nm or less to make itpossible to permeate light, light can be emitted from a second pixelelectrode 17 side as indicated by arrow in the drawing.

FIG. 8E shows an example of emitting light from both of a firstelectrode and a second electrode. A first pixel electrode 11 is formedby a conductive film having a light transmitting property and a highwork function and a second pixel electrode 17 is formed by a conductivefilm having a light transmitting property and a low work function.Typically, the first pixel electrode 11 is formed of an oxide conductivematerial including a silicon oxide at a concentration of 1 to 15 atomic% and the second electrode 17 is composed of a third electrode layer 33containing an elemental substance of an alkali metal or an alkali earthmetal such as LiF or CaF or a compound of an alloy thereof, with athickness of 100 nm or less and a fourth electrode layer 34 formed of ametal material such as aluminum with a thickness of 100 nm or less.Accordingly, light can be emitted from both of the first pixel electrode11 and the second electrode 17 as indicated by an arrow in the drawing.

FIG. 8C shows an example of forming a first pixel electrode 11 by usinga conductive film having a light transmitting property and a low workfunction and forming a second pixel electrode 17 by a conductive filmhaving a high work function. A structure of a layer containing a lightemitting substance is illustrated as a stacked layer structure formed bystacking sequentially an electron transporting or injecting layer 43, alight emitting layer 42, and a hole injecting or transporting layer 41.The second pixel electrode 17 is composed of a second electrode layer 32formed of an oxide conductive material containing silicon oxide at aconcentration of 1 to 15 atomic %, and a first electrode layer 35 formedof a metal such as aluminum or titanium, or a metal and a metal materialcontaining nitrogen at a concentration of a stoichiometric compositionratio or less. The first pixel electrode 11 is composed of a thirdelectrode layer 33 containing an elemental substance of an alkali metalor an alkali earth metal such as LiF or MgAg or a compound of an alloythereof, and a fourth electrode layer 34 formed of a metal material suchas aluminum. By forming each the third electrode layer 33 and the fourthelectrode layer 34 to have a thickness of 100 nm or less to make itpossible to permeate light, light can be emitted from the firstelectrode 11 side as indicated by an arrow in the drawing.

FIG. 8D shows an example of forming a first pixel electrode 11 by usinga conductive film having a low work function and forming a second pixelelectrode 17 by using a conductive film having a light transmittingproperty and a high work function. A structure of a layer containing alight emitting substance is illustrated as a stacked layer structureformed by stacking sequentially an electron transporting or injectinglayer 43, a light emitting layer 42, and a hole injecting ortransporting layer 41. The first pixel electrode 11 is formed to have asimilar structure to that illustrated in FIG. 8A and to have a thicknessthat enables it to reflect light generated in the layer containing alight emitting substance. The second pixel electrode 17 is formed of anoxide conductive material containing silicon oxide at a concentration of1 to 15 atomic %. By forming a hole injecting layer by a metal oxidewhich is an inorganic material (typically, molybdenum oxide or vanadiumoxide), oxygen which is introduced when forming the second electrodelayer 32 is supplied and a hole injecting property is improved,accordingly, drive voltage can be reduced in this structure. By formingthe second electrode 17 by a conductive film having a light transmittingproperty, light can be emitted from one side of the second electrode 17as indicated by an arrow.

FIG. 8F shows an example of emitting light from both sides, that is, afirst pixel electrode and a second pixel electrode. A first pixelelectrode 11 is formed by a conductive film having a light transmittingproperty and a low work function and a second pixel electrode 17 isformed by a conductive film having a light transmitting property and ahigh work function. Typically, the first electrode 11 is composed of athird electrode layer 33 containing an elemental substance of an alkalimetal or an alkali earth metal such as LiF or CaF or a compound or analloy thereof, with a thickness of 100 nm or less and a fourth electrodelayer 34 formed of a metal material such as aluminum with a thickness of100 nm or less. The second pixel electrode 17 is formed of an oxideconductive material containing a silicon oxide at a concentration of 1to 15 atomic %.

The layer containing a light emitting substance 16 can be formed by acharge injection transportation material and a light emitting materialincluding an organic compound or an inorganic compound, can include oneor a plurality types of layers selected from a low molecular organiccompound, an intermolecular organic compound (which does not have asubliming property but have a molecular chain length of 10 μm or less astypified by dendrimer, oligomer, or the like), and a high molecularorganic compound, and can be combined with an inorganic compound havingan electron injecting transporting property or a hole injectingtransporting property.

As a particularly high electron transporting material among chargeinjection transporting materials, for example, metal complexes having aquinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), andbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq) can be given.

As a high hole transporting material, for example, aromatic amine basedcompounds (i.e., one having a benzene ring-nitrogen bond), such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA); and4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA) can be given.

As a particularly high electron injecting material among chargeinjection transportation materials, compounds of alkali metal oralkaline earth metal such as lithium fluoride (LiF), cesium fluoride(CsF), and calcium fluoride (CaF₂) can be given. In addition, a mixtureof a highly electron transporting material such as Alq₃ and alkalineearth metal such as magnesium (Mg) may be used.

As a highly hole injecting material among charge injectiontransportation materials, for example, a metal oxide such as molybdenumoxide (MoO_(x)), vanadium oxide (VO_(x)), ruthenium oxide (RuO_(x)),tungsten oxide (WO_(x)), or manganese oxide (MnO_(x)) can be given.Besides these, phthalocyanine based compounds such as phthalocyanine(H₂Pc) and copper phthalocyanine (CuPc) can be given.

Light emitting layers 42 having different light emission wavelengthbands may be each formed in pixels so as to perform color display.Typically, light emitting layers corresponding to respective luminescentcolors of R (red), G (green), and B (blue) are formed. In this case,color purity can be improved and specular reflection (glare) of a pixelportion can be prevented by providing a filter (coloring layer) thattransmits light of a certain light emission wavelength band on a lightemission side of the pixels. By providing the filter (coloring layer), acircular polarizing plate or the like, which has been conventionallythought to be required, can be omitted, thereby reducing loss of lightemitted from the light emitting layers. In addition, a change in hue,which is caused in the case where a pixel portion (a display screen) isseen obliquely, can be reduced.

There are various kinds of light emitting materials that can be used forforming the light emitting layers 42. With respect to low molecularorganic light emitting materials, the following substances can be used:4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCTT);2-tert-butyl-4-dicyanomethylene-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTB); periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridone (abbreviation: DMQd); coumarin 6; coumarin545T; tris(8-quinolinolato)aluminum (abbreviation: Alq₃);9,9′-bianthryl; 9,10-diphenylanthracene (abbreviation: DPA);9,10-bis(2-naphthyl)anthracene (abbreviation: DNA); and the like. Also,another substance may be used.

On the other hand, a high molecular organic light emitting material hashigher physical strength than that of a low molecular organic lightemitting material, and so a light emitting element formed of a highmolecular organic material has high durability. Since a high molecularorganic light emitting material can be formed into a film by coating,manufacturing an element is relatively easy. A light emitting elementstructure using the high molecular organic light emitting material isbasically the same as that formed by a low molecular organic lightemitting material formed by stacking sequentially a cathode, a layercontaining a light emitting substance, and an anode. However, a stackedlayer structure which is formed in the case of using a low molecularorganic light emitting material is difficult to be formed as a stackedlayer structure composed of a layer containing a light emittingsubstance formed of a high molecular organic light emitting material.Most cases, the layer containing a light emitting substance is formed tohave two stacked layers. Specifically, a structure is composedsequentially of a substrate, a layer containing a light emittingsubstance, a hole transporting layer, and an anode.

Since emission color is determined by a material for forming the lightemitting layer, a desired light emitting element exhibiting desiredlight emission can be formed by selecting the material. As a highmolecular light emitting material which can be used for forming thelight emitting layer, polyparaphenylene vinylene based,polyparaphenylene based, polythiophene based, and polyfluorene basedmaterials can be given.

As the polyparaphenylene vinylene based material, a derivative ofpoly(paraphenylenevinylene) (PPV):poly(2,5-dialkoxy-1,4-phenylenevinylene) (RO-PPV);poly(2-(2′-ethyl-hexoxy)-5-methoxy-1,4-phenylenevinylene) (MEH-PPV);poly(2-(dialkoxyphenyl)-1,4-phenylenevinylene) (ROPh-PPV); or the likecan be given. As the polyparaphenylene based material, a derivative ofpolyparaphenylene (PPP): poly(2,5-dialkoxy-1,4-phenylene) (RO-PPP);poly(2,5-dihexoxy-1,4-phenylene); or the like can be given. As thepolythiophene based material, a derivative of polythiophene (PT):poly(3-alkylthiophene) (PAT); poly(3-hexylthiophene) (PHT);poly(3-cyclohexylthiophene) (PCHT); poly(3-cyclohexyl-4-methylthiophene)(PCHMT); poly(3,4-dicyclohexylthiophene) (PDCHT);poly[3-(4-octylphenyl)-thiophene](POPT);poly[3-(4-octylphenyl)-2,2bithiophene) (PTOPT); or the like can begiven. As the polyfluorene based material, a derivative of polyfluorene(PF): poly(9,9-dialkylfluorene) (PDAF); poly(9,9-dioctylfluorene)(PDOF); or the like can be given.

In the case that a high molecular organic light emitting material havinga hole transporting property is interposed between an anode and a highmolecular organic light emitting material having a light emittingproperty, a hole injecting property of the anode can be improved.Generally, the one which is dissolved with an acceptor material intowater is applied by a spin coating method or the like. Since the highmolecular organic light emitting material having the hole transportingproperty is insoluble in an organic solvent, the foregoing material canbe stacked over the above mentioned light emitting material having alight emitting property. As the high molecular organic light emittingmaterial having a hole transporting property, a mixture of PEDOT andcamphor sulfonic acid (CSA) as an acceptor material; a mixture ofpolyaniline (PANI) and polystyrenesulfonic acid (PSS) as an acceptormaterial; or the like can be given.

The light emitting layers 42 can be formed to have a structureexhibiting a single color emission or white emission. In the case ofusing a white light emitting material, color display can be realized byproviding a filter (coloring layer) transmitting light at a specifiedwavelength at a light emission side of a pixel.

In order to form a light emitting layer emitting white emission, Alq₃,Alq₃ doped partly with Nile red which is a red emission coloring matter,Alq₃, p-EtTAZ, and TPD (aromatic diamine) are stacked sequentially by avapor deposition method. In the case of forming a light emitting layerby a coating method using spin coating, the foregoing material ispreferably coated and baked by vacuum heating. For example, an aqueoussolution of poly(ethylene dioxythiophene)/poly(styrenesulfonic acid)(PEDOT/PSS), which functions as a hole injecting layer, may be appliedover an entire surface of a substrate and baked. Afterwards, a solutionof polyvinyl carbazole (PVK) doped with a luminescence center pigment(such as 1,1,4,4-tetraphenyl-1,3-butadiene (TPB),4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM1),Nile red, or coumarin 6), which serves as a light emitting layer, maythen be applied over the entire surface and baked.

The light emitting layer can be formed by a single layer.1,3,4-oxadiazole derivatives (PBD) having an electron transportingproperty can be dispersed to polyvinylcarbazole (PVK) having a holetransporting property. Further, white emission can be obtained bydispersing PBD of 30 wt % as an electron transporting agent anddispersing an appropriately amount of four kinds coloring matters (TPB,coumarin 6, DCM1, and Nile red). Besides the light emitting elementexhibiting white emission, light emitting elements exhibiting redemission, green emission, or blue emission can be manufactured byappropriately selecting a material of the light emitting layer.

In the case that a high molecular organic material having a holetransporting property is interposed between an anode and a highmolecular organic material having a light emitting property, a holeinjecting property of the anode can be improved. Generally, a highmolecular organic material having a hole transporting property dissolvedin water together with an acceptor material is coated by a spin coatingmethod. Since the high molecular organic material having a holetransporting property is insoluble in an organic solvent, the foregoingmaterial can be stacked over the above mentioned light emitting materialhaving a light emitting property. As the high molecular organic materialhaving a hole transporting property, a mixture of PEDOT and camphorsulfonic acid (CSA) as an acceptor material; a mixture of polyaniline(PANI) and polystyrenesulfonic acid (PSS) as an acceptor material; orthe like can be given.

As a material for the light emitting layers 42, a triplet excitedmaterial including metal complexes can be used besides a singlet excitedlight emitting material. For example, a red luminescent pixel which hasa relatively short half-brightness life is formed by a triplet excitedlight emitting material among the red luminescent pixel, a greenluminescent pixel, and blue luminescent pixel; and the other pixels areformed by a singlet excited light emitting material. Since a tripletexcited light emitting material has good emission efficiency, there isan advantage of obtaining luminescence which can be obtained in the caseof using a singlet excited light emitting material at low powerconsumption. That is, reliability can be improved since a light emittingelement can be operated at a small amount of current in the case ofapplying a triplet excited light emitting material for a red emissionpixel. In order to reduce power consumption, a red luminescent pixel anda green luminescent pixel are

formed by a triplet excited light emitting material, and a blueluminescent pixel can be formed by a singlet excited light emittingmaterial. By forming a green luminescent pixel which is well visible forhuman by a triplet excited light emitting material, power consumptioncan be further reduced.

As an example of a triplet excited light emitting material, a materialusing metal complexes as a dopant can be nominated. The following areknown as the foregoing metal complexes: metal complexes having platinumwhich is the third transition series element as a central metal, metalcomplexes having iridium as a central metal, or the like. Thesecompounds are not limited as a triplet excited light emitting material.A compound having the foregoing structure and a compound having Group 8to Group 10 elements as a central metal can be used.

The following materials for forming the foregoing layer containing alight emitting substance are illustrative only. A light emitting elementcan be formed by appropriately stacking functional each layer such as ahole injecting transporting layer, a hole transporting layer, anelectron injecting transporting layer, an electron transporting layer, alight emitting layer, an electron blocking layer, or a hole blockinglayer. Further, a mixed layer or mixed junction can be formed bycombining each of the foregoing layers. A layer structure of the lightemitting layer is variable. Instead of not providing a specific electroninjection region or light emitting region, various changes andmodifications such as providing an electrode or a dispersed luminescentmaterial for being used only for the electron injection region or thelight emitting region are permissible unless otherwise such changes andmodifications depart from the scope of the present invention.

Embodiment 2

In this embodiment, a pixel circuit of a display panel of a lightemitting device relating to the present invention and an operationalconfiguration thereof is described with reference to FIGS. 9A to 9F. Forthe operational configuration of the display panel in a display devicein which video signals are digital, there is a configuration in whichvideo signals to be input to a pixel is regulated by voltage, and aconfiguration in which they are regulated by current. As theconfiguration in which video signals are regulated by voltage, there isone where voltage applied to a light emitting element is constant(CVCV), and one where current applied to the light emitting element isconstant (CVCC). Also, as the configuration in which video signals areregulated by current, there is one where voltage applied to the lightemitting element is constant (CCCV), and one where current applied tothe light emitting element is constant (CCCC). This embodiment describesa pixel of a CVCV operation with reference to FIGS. 9A and 9B. Further,a pixel of a CVCC operation is described with reference to FIGS. 9C to9F.

In the pixel shown in each of FIGS. 9A and 9B, a signal line 3710 and apower source line 3711 are arranged in a column direction and a scanningline 3714 is arranged in a row direction. Also, a switching TFT 3701, adriving TFT 3703, a capacitor element 3702, and a light emitting element3705 are included.

Note that the switching TFT 3701 and the driving TFT 3703 are operatedin a linear region when they are turned on. Also, the driving TFT 3703has a role of controlling whether voltage is applied to the lightemitting element 3705. It is favorable in terms of a manufacturing stepif both TFTs have the same conductivity type. In this embodiment, theswitching TFT 3701 is formed as an n-channel type TFT, and the drivingTFT 3703 is formed as a p-channel type TFT. Also, as the driving TFT3703, a depletion type TFT may be used in addition to an enhancementtype TFT. Further, a ratio (W/L) of a channel width W and a channellength L of the driving TFT 3703 is preferably 1 to 1000, even though itdepends on a mobility of the TFT. As W/L gets larger, an electricalproperty of the TFT is improved.

In the pixel shown in each of FIGS. 9A and 9B, the switching TFT 3701controls input of video signals to the pixel, and when the switching TFT3701 is turned on, video signals are input inside the pixel. Then,voltage of the video signals is retained in the capacitor element 3702.

In FIG. 9A, in a case where the power source line 3711 is Vss and anopposing electrode of the light emitting element 3705 is Vdd, as inFIGS. 8C and 8D, the opposing electrode of the light emitting element isan anode, and an electrode connected to the driving TFT 3703 is acathode. In this case, luminance irregularity due to characteristicvariation of the driving TFT 3703 can be suppressed.

In FIG. 9A, in a case where the power source line 3711 is Vdd and theopposing electrode of the light emitting element 3705 is Vss, as inFIGS. 8A and 8B, the opposing electrode of the light emitting element isa cathode, and the electrode connected to the driving TFT 3703 is ananode. In this case, by inputting video signals having higher voltagethan Vdd to the signal line 3710, voltage of the video signals areretained in the capacitor element 3702 and the driving TFT 3703 operatedin the linear region; consequently, luminance irregularity due tovariation of the TFT can be improved.

The pixel shown in FIG. 9B has the same pixel configuration as thatshown in FIG. 9A except that in FIG. 9B, a TFT 3706 and a scanning line3715 are added.

Turning on or off of the TFT 3706 is controlled by the newly placedscanning line 3715. When the TFT 3706 is turned on, a charge retained inthe capacitor element 3702 is discharged, and the driving TFT 3703 isturned off. In other words, according to a placement of the TFT 3706, astate in which current is not fed to the light emitting element 3705 canbe created forcefully. Therefore, the TFT 3706 can be called an erasingTFT. Consequently, in the configuration in FIG. 9B, a duty ratio oflight emission can be improved since a lighting period can be started atthe same time as or right after a start of a writing period, withoutwaiting for signals to be written to all pixels.

In a pixel having the foregoing operational configuration, a currentvalue of the light emitting element 3705 can be determined by thedriving TFT 3703 which operates in the linear region. By the foregoingconfiguration, characteristic variation of TFTs can be suppressed,luminance irregularity of light emitting elements due to thecharacteristic variations of the TFTs can be improved, and a displaydevice with improved image quality can be provided.

Next, a pixel of a CVCC operation is described with reference to FIGS.9C to 9F. The pixel shown in FIG. 9C has a pixel configuration shown inFIG. 9A with a power source line 3712 and a current control TFT 3704provided in addition.

The pixel shown in FIG. 9E has the same configuration as the pixel shownin FIG. 9C, except that a gate electrode of the driving TFT 3703 isconnected to the power supply line 3712 arranged in a row direction. Inother words, both pixels shown in FIGS. 9c and 9E show the sameequivalent circuit schematic. However, the power supply line 3712arranged in a column direction (FIG. 9C) is formed with a conductivefilm formed in a different layer from that of the power supply line 3712arranged in a row direction (FIG. 9E). Here, wirings to which the gateelectrode of the driving TFT 3703 are connected is given focus, and inorder to show that layers for manufacturing the wirings are different,they are separately described in FIGS. 9C and 9E.

Note that the switching TFT 3701 operates in the linear region, and thedriving TFT 3703 operates in a saturation region. Also, the driving TFT3703 has a role of controlling a current value fed to the light emittingelement 3705, and the current control TFT 3704 operates in thesaturation region has a role of controlling supply of current to thelight emitting element 3705.

The pixel shown in each of FIGS. 9D and 9F have the same pixelconfiguration as the pixel shown in each of FIGS. 9C and 9E,respectively, except that they are each provided with an erasing TFT3706 and the scanning line 3715 in addition.

Note that in the pixels shown in FIGS. 9A and 9B, CVCC operations arealso possible. Also, for pixels having the operational configurationsshown in FIGS. 9C to 9F, respectively, similarly to FIGS. 9A and 9B, Vddand Vss can be appropriately changed depending on a direction in whichcurrent of a light emitting element flows.

In a pixel having the foregoing configuration, since the current controlTFT 3704 operates in the linear region, a small shift in Vgs of thecurrent control TFT 3704 does not have an effect on the current value ofthe light emitting element 3705. In other words, the current value ofthe light emitting element 3705 can be determined by the driving TFT3703 which operated in the saturation region. By the foregoingconfiguration, luminance irregularity of light emitting elements due tocharacteristic variations of TFTs can be improved, and a display devicewith improved image quality can be provided

Note that although a configuration in which the capacitor element 3702is provided is shown, the present invention is not limited thereto, andin a case where a capacity for retaining video signals can be covered bya gate capacitance, the capacitor element 3702 is not required to beprovided.

By such an active matrix type display device, in a case where pixeldensity is increased, low voltage drive is possible since a TFT isprovided in each pixel, and this is considered to be advantageous.

Further, in a display device relating to the present invention, adriving method of a screen display is not particularly limited, and forexample, a dot sequential driving method, a line sequential drivingmethod, an area sequential driving method, or the like may be used.Typically, the line sequential driving method is used, and a timedivision gray scale driving method or an area gray scale driving methodmay be appropriately used. Further, image signals input to a source lineof the display device may be analog signals, or digital signals, and adriver circuit and the like may be designed appropriately according tothe image signals.

Embodiment 3

In this embodiment, mounting of a driver circuit relating to the presentinvention is described with reference to FIGS. 10A to 10C.

As shown in FIG. 10A, a signal line driver circuit 1402 and scanningline driver circuits 1403 a and 1403 b are mounted on a periphery of apixel portion 1401. In FIG. 10A, as the signal line driver circuit 1402and the scanning line driver circuits 1403 a and 1403 b, an IC chip 1405is mounted on a substrate 1400 by a known mounting method such as amethod using an anisotropic conductive adhesive or an anisotropicconductive film, a COG method, a wire bonding, a reflow treatment usinga solder bump, or the like. Here, the IC chip 1405 is mounted by a COGmethod, and connected to an external circuit through an FPC (flexibleprinted circuit) 1406.

In a case where a semiconductor element typified by a TFT is formed withan oxide semiconductor as shown in FIG. 10B, the pixel portion 1401, thescanning line driver circuits 1403 a and 1403 b, and the like may beintegrated over the substrate while the signal line driver circuit 1402and the like may be separately mounted as IC chips. In FIG. 10B, the ICchip 1405 as the signal line driver circuit 1402 is mounted on thesubstrate 1400 by a COG method. The IC chip 1405 is connected to anexternal circuit through the FPC 1406.

Further, as shown in FIG. 10C, there is a case where the signal linedriver circuit 1402 and the like are mounted by a TAB method instead ofa COG method. The IC chip is connected to an external circuit throughthe FPC 1406. Although the signal line driver circuit is mounted by aTAB method in FIG. 10C, the scanning line driver circuit may be mountedby a TAB method.

When the IC chip is mounted by a TAB method, the pixel portion canoccupy a large area in the substrate, leading to a narrower frame.

Instead of an IC chip formed over a silicon wafer, an IC (hereinafterreferred to as a driver IC) formed over a glass substrate may beprovided. Since an IC chip is formed over a circular silicon wafer, theshape of a mother substrate is limited. Meanwhile, a driver IC is formedover a glass substrate whose shape is not limited, which results inincreased productivity. Accordingly, the shape and size of a driver ICcan be set freely. For example, when forming a driver IC with a longside of 15 to 80 mm, a smaller number of driver ICs are required ascompared to the case of mounting IC chips. As a result, the number ofconnection terminals can be reduced and productive yield can beincreased.

A driver IC can be formed using a crystalline semiconductor formed overa substrate, and the crystalline semiconductor may be formed bycontinuous wave laser light irradiation. A semiconductor film obtainedby continuous wave laser light irradiation has few crystal defects andlarge crystal grains. Accordingly, a transistor having such asemiconductor film is improved in mobility and response speed, capableof high speed driving, and suitable for a driver IC. A driver IC may beformed using an oxide semiconductor film of the present invention inwhich crystallinity of at least a channel forming region is improved.

Embodiment 4

In this embodiment, a display module relating to the present inventionis described. Here, as one example of the display module, a liquidcrystal module is described with reference to FIG. 11.

A substrate 1601 and an opposing substrate 1602 are stuck together by asealant 1600, and a pixel portion 1603 and a liquid crystal layer 1604are provided therebetween to form a display region.

A coloring layer 1605 is required in a case of performing color display,and in a case of an RGB method, a coloring layer corresponding to eachof red, green and blue are provided corresponding to each pixel. On theoutsides of the substrate 1601 and the opposing substrate 1602,polarizing plates 1606 and 1607 are provided, respectively. Also, on asurface of the polarizing plate 1606, a protective film 1616 is formed,and alleviates impact from the exterior.

A wiring substrate 1610 is connected to a connection terminal 1608provided over the substrate 1601 via an FPC 1609. External circuits 1612such as a pixel driver circuit (an IC chip, a driver IC, or the like), acontrol circuit, a power source circuit or the like is incorporated tothe wiring substrate 1610.

A cold cathode tube 1613, a reflecting plate 1614, and an optical film1615 are a backlight unit, and these become a light source to emit lightto a liquid crystal display panel. A liquid crystal panel, the lightsource, the wiring substrate, the FPC, and the like are retained andprotected in a bezel 1617.

Embodiment 5

In this embodiment mode, as an electronic appliance relating to thepresent invention, a television device (also simply called a TV, or atelevision receiving device), a digital camera, a digital video camera,a mobile phone device (also simply called a cellular phone device or acellular phone), a mobile information terminal such as a PDA, a mobilegame machine, a monitor for a computer, a computer, an audio reproducingdevice such as a car audio component, an image reproducing device suchas a home-use game machine provided with a recording medium, or thelike, is described with reference to drawings.

The mobile information terminal shown in FIG. 12A includes a main body9201, a display portion 9202, and the like. By using a display devicethat is one feature of the present invention, the mobile informationterminal can be provided inexpensively.

The digital video camera shown in FIG. 12B includes a display portion9701, a display portion 9702, and the like. By using the display devicethat is one feature of the present invention, the digital video cameracan be provided inexpensively.

The mobile terminal shown in FIG. 12C includes a main body 9101, adisplay portion 9102, and the like. Embodiment Modes 1 to 5, andembodiments 1 to 4 can be applied to the display portion 9102. By usingthe display device that is one feature of the present invention, themobile terminal can be provided inexpensively.

The mobile type television device shown in FIG. 12D includes a main body9301, a display portion 9302, and the like. By using the display devicethat is one feature of the present invention, the mobile type televisiondevice can be provided inexpensively. The present invention can bewidely applied to a small scale television device such as a televisiondevice mounted on a mobile terminal such as a cellular phone, a mediumscale television device that can be carried around, and a large scaletelevision device (for example, 40-inch or larger).

The mobile type computer shown in FIG. 12E includes a main body 9401, adisplay portion 9402, and the like. By using the display device that isone feature of the present invention, the mobile type computer can beprovided inexpensively.

The television device shown in FIG. 12F includes a main body 9501, adisplay portion 9502, and the like. By using the display device that isone feature of the present invention, the television device can beprovided inexpensively.

Among the foregoing electronic appliances, that which uses a secondarybattery can have a longer operating time by how much power consumptionis reduced, and a need for recharging the secondary battery can be cutout.

Embodiment 6

In this embodiment, a structure of an LRTA device used in the presentinvention is described with reference to FIGS. 19A and 19B.

In FIG. 19A, a gate electrode 1922, a gate insulating films 1923 a and1923 b, and an oxide semiconductor film 1902 are formed over a glasssubstrate 1901. Also, on a lower surface side of the substrate and on anupper surface side of the substrate, an infrared light lamp 1903 and anultraviolet light lamp 1904 are provided, respectively. And, a firstinfrared light auxiliary lamp 1905, and a second infrared lightauxiliary lamp 1906 are provided in parallel with the ultraviolet lightlamp 1904. Note that the first infrared light auxiliary lamp 1905 andthe second infrared light auxiliary lamp 1906 are not required to beprovided.

Also, this embodiment mode has a structure in which the first infraredlight auxiliary lamp 1905 and the second infrared light auxiliary lamp1906 are placed in front and in back (with respect to a moving directionof the substrate) of the ultraviolet light lamp 1904, respectively.However, the structure may be that both are placed in the front or inthe back.

In a structure such as the above, each lamp (the infrared light lamp1903, the ultraviolet light lamp 1904, the first infrared lightauxiliary lamp 1905, and the second infrared light auxiliary lamp 1906)moves in a direction of an arrow in FIG. 19A, and scans a linear light.In the structure of this embodiment, a region 1908 shown by a dottedline in the oxide semiconductor film 1902 that overlaps with the gateelectrode 1922 with the gate insulating films 1923 a and 1923 btherebetween is irradiated with infrared light from the first infraredlight auxiliary lamp 1905 to be heated. Note that each lamp is movedwhen lamp irradiation is performed on the substrate; however, the glasssubstrate may be moved, or both the lamp and the substrate may be moved.

After irradiation is performed on the first infrared light auxiliarylamp 1905, the upper surface side of the substrate is irradiated withultraviolet light from the ultraviolet light lamp 1904, as well as thelower surface side of the substrate is irradiated with infrared lightfrom the infrared light lamp 1903, and the region 1908 of the oxidesemiconductor film 1902 that overlaps with the gate electrode 1922 isheated. In this embodiment, crystallization of the oxide semiconductorfilm 1902 is performed with this region 1908 having priority.

The region 1908 heated by irradiation with the ultraviolet light lamp1904 and the infrared light lamp 1903 is heated with infrared light fromthe second infrared light auxiliary lamp 1906 that is placed in back ofthe ultraviolet light lamp 1904. Irradiation with infrared light fromthe second infrared light auxiliary lamp 1906 is provided to furtherheat the region 1908 in which crystallization is promoted.

As in the foregoing, the region 1908 of the oxide semiconductor film1902 (the region that becomes a crystalline oxide semiconductor film bya crystallization step) that overlaps with the gate electrode 1922appears to move to the front along with a movement of the substrate.

FIG. 19B shows a graph showing a relationship between time (Time) andtemperature (Temp.) of the region 1908 of the oxide semiconductor film1902. As shown in FIG. 19B, the region 1908 comes to a preheating state,then continues on to a main heating state, and a post heating state,with passing of time.

As clear from FIG. 19B, in the preheating state, a temperature is raisedto a certain degree so that a temperature gradient with the subsequentmain heating state is alleviated. This is so that accumulation ofwarping energy and the like in the oxide semiconductor film due to beingheated suddenly in the main heating state, is prevented.

Therefore, it is desirable that output energy of the first infraredlight auxiliary lamp 1905 is set to be smaller than output energy of theinfrared light lamp 1903. At this time, a practitioner may decide howadjustment is to be made to form the appropriate temperature gradient.

Next, after the preheating state, infrared light irradiation isperformed towards a lower surface side of the substrate, and the oxidesemiconductor film 1902 is brought to the main heating state in which afilm surface temperature is raised to 250° C. to 570° C. At this state,crystallinity of the region 1908 in the oxide semiconductor film 1902becomes favorable. Note that ultraviolet light emitted at the same timecontributes to electron excitation; therefore, it does not contribute tochange in terms of heat.

The region 1908 with improved crystallinity obtained in the main heatingstate is heated by the second infrared auxiliary lamp 1906 placed inback of the ultraviolet light lamp 1904. This post heating state has arole of preventing a completion of crystallization in a state wherethermal equilibrium is deteriorated by sudden cooling in the mainheating state. This is a device for obtaining the most stable bond stateby providing allowance in a time period required for crystallization.

Accordingly, it is desirable that output energy of the second infraredlight auxiliary lamp 1906 is also set to be smaller than that ofinfrared light lamp 1903 placed under a substrate surface, and adjustedso that a temperature gradient is formed in which the temperature isgradually lowered.

By a structure as in the foregoing, shrinking of a substrate can besuppressed since a portion of an oxide semiconductor film that overlapswith a gate electrode is heated. Also, by performing crystallization bymoving each lamp or substrate, throughput can be increased. Also,occurrence of a crystal defect such as stress warping, a dangling bond,or the like that can occur due to sudden heating of an oxidesemiconductor film or sudden cooling of a crystalline oxidesemiconductor film can be suppressed, and the oxide semiconductor filmincluding the region 1908 with excellent crystallinity can be obtained.

Also, by performing irradiation heating without providing the firstinfrared light auxiliary lamp 1905 and the second infrared lightauxiliary lamp 1906, heating of the substrate may be suppressed.

Note that in this embodiment, a structure of an LRTA device using alinear lamp is described; however, a planar lamp may be used to performthe crystallization step.

Embodiment 7

In this embodiment, an example of applying a semiconductor devicerelating to the present invention to an electrophoresis display deviceis described with reference to FIG. 20.

The electrophoresis display device shown in FIG. 20 includes a main body2010, a pixel portion 2011 displaying an image, a driver IC 2012, areceiving device 2013, a film battery 2014, and the like. Each of thedriver IC 2012, the receiving device 2013, and the like may be mountedwith a semiconductor part. The semiconductor device of the presentinvention can be used for the pixel portion 2011 and the driver IC 2012.Note that the pixel portion 2011 has a structure where a display layerin which microcapsules, Gyricon beads, and the like are arranged and adriver layer controlling the display layer are stacked. The displaylayer and the driver layer are interposed between two plastic films.

Such an electrophoresis display device is also called an electronicpaper, and it is extremely light weight, and since it has a flexibleproperty, it can be rolled up in a tubular form; consequently, it isextremely advantageous in carrying around. Therefore, a display mediumof a large screen can be freely carried around. Also, since thesemiconductor of the present invention is used for the pixel portion2011 and the like, an inexpensive display device can be provided.

A variety of modes can be considered as an electrophoresis displaydevice of this embodiment, but the electrophoresis display device ofthis embodiment is a device in which a plurality of microcapsules eachincluding first particles having a positive charge and second particleshaving a negative charge are dispersed in a solvent or a solute, and anelectrical field is applied to the microcapsules so that the particlesin the microcapsules move in opposite directions of each other, and onlya color of the particles gathered on one side is displayed. Note thatthe first particles or the second particles includes a colorant, anddoes not move in a case where there is not electric field. Also, a colorof the first particles is different from a color of the second particles(the particles may also be colorless). That which microcapsules aredispersed in a solvent is called an electronic ink, and this electronicink can be printed on a surface such as glass, plastic, fabric, paper,and the like.

Also, in a semiconductor device of the present invention, in addition toan oxide semiconductor film having a light transmitting property withrespect to visible light, a transparent conductive film including indiumtin oxide (ITO), ITSO made of indium tin oxide and silicon oxide,organic indium, organic tin, zinc oxide, titanium nitride, or the likeeach having a light transmitting property with respect to visible lightfor a source electrode, a drain electrode, and the like. If aconventional amorphous silicon or polysilicon is used for a TFT used ina driver layer, to prevent a channel forming region from beingirradiated with light, it is necessary that a light shielding film isprovided to overlap the channel forming region. However, as in thepresent invention, by manufacturing the driver layer using the oxidesemiconductor film, the source electrode, and the drain electrode eachhaving a light transmitting property with respect to visible light, anelectrophoresis display device of a double-sided display can beobtained.

Note that the semiconductor device of the present invention can be usedas a means for displaying mainly still images for a navigation system,an audio reproducing device (such as a car audio component, or an audiocomponent), a personal computer, a game machine, a mobile informationterminal (such as a mobile computer, a cellular phone, a mobile gamemachine, or an electronic book), and in addition, the semiconductordevice can be used for household appliances such as a refrigerator, awashing machine, a rice cooker, a fixed telephone, a vacuum cleaner, anda clinical thermometer, as well as for a hanging poster in a train, anda large-sized information display such as an arrival and departure guideboard in a railroad station and an airport.

Embodiment 8

In this embodiment, a digital audio player relating to the presentinvention is described with reference to FIG. 21.

The digital audio player shown in FIG. 21 includes a main body 2110, adisplay portion 2111, a memory portion 2112, an operation portion 2113,a pair of earphones 2114, and the like. Note that instead of the pair ofearphones 2114, a pair of headphones, or a wireless pair of earphonescan be used. As the display portion 2111, liquid crystal, organic EL, orthe like can be used. As the memory portion 2112, a flash memory with arecording capacity of 200 megabytes (MB) to 200 gigabytes (GB) is used,and by operating the operation portion 2113, an image or a sound (music)can be recorded and reproduced.

Since a channel forming region of an oxide semiconductor film of a TFTincluded in a semiconductor device of the present invention includes atleast a crystallized region, by providing the semiconductor device ofthe present invention to the display portion 2111, an inexpensivedigital audio player with good performance can be provided. Further,since the channel forming region of the oxide semiconductor film istransparent and does not absorb visible light, unnecessary lightcarriers are not generated. Therefore, since characteristic degradationof the channel forming region due to light irradiation does not occur, ahighly reliable digital audio player can be provided.

This embodiment can be appropriately combined with Embodiment Modes 1 to6 and Embodiments 1 to 4.

This application is based on Japanese Patent Application serial no.2005-283782 filed in Japan Patent Office on Sep. 29, in 2005, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising the steps of: forming a gate electrode over a substrate;forming a first insulating film comprising silicon nitride over the gateelectrode; forming a second insulating film comprising silicon andoxygen over the first insulating film; forming an oxide semiconductorfilm over the gate electrode with the first insulating film and thesecond insulating film interposed therebetween, the oxide semiconductorfilm including a channel formation region; forming a source electrodeand a drain electrode over the oxide semiconductor film, wherein part ofthe oxide semiconductor film between the source electrode and the drainelectrode is etched; and forming a third insulating film over the oxidesemiconductor film, the source electrode and the drain electrode, thethird insulating film comprising silicon oxide and being in contact withat least a portion of the oxide semiconductor film, wherein each of thesource electrode and the drain electrode comprises a nitride of a metalselected from the group consisting of Al, W, Mo, Ta, and Ti, and whereinthe oxide semiconductor film comprises gallium, zinc and indium.
 2. Themethod according to claim 1, wherein each of the first insulating filmand the oxide semiconductor film is formed by sputtering.
 3. The methodaccording to claim 1, further comprising the step of heating the oxidesemiconductor film at a temperature 250° C. to 570° C. in order tocrystallize at least a portion of the oxide semiconductor film.
 4. Themethod according to claim 1, further comprising the step ofcrystallizing at least a portion of the oxide semiconductor film bylaser irradiation.
 5. The method according to claim 1, furthercomprising the step of crystallizing at least a portion of the oxidesemiconductor film by rapid thermal annealing using light.
 6. A methodof manufacturing a semiconductor device comprising the steps of: forminga gate electrode over a substrate; forming a first insulating filmcomprising silicon oxide over the gate electrode; forming an oxidesemiconductor film over the gate electrode with the first insulatingfilm interposed therebetween, the oxide semiconductor film including achannel formation region; forming a source electrode and a drainelectrode over the oxide semiconductor film, wherein part of the oxidesemiconductor film between the source electrode and the drain electrodeis etched; and forming a second insulating film over the oxidesemiconductor film, the source electrode and the drain electrode,wherein the second insulating film is in contact with a surface of theoxide semiconductor film, wherein each of the source electrode and thedrain electrode comprises a nitride of a metal selected from the groupconsisting of Al, W, Mo, Ta, and Ti, and wherein the oxide semiconductorfilm comprises gallium, zinc and indium.
 7. The method according toclaim 6, wherein each of the first insulating film and the oxidesemiconductor film is formed by sputtering.
 8. The method according toclaim 6, further comprising the step of heating the oxide semiconductorfilm at a temperature 250° C. to 570° C. in order to crystallize atleast a portion of the oxide semiconductor film.
 9. The method accordingto claim 6, further comprising the step of crystallizing at least aportion of the oxide semiconductor film by laser irradiation.
 10. Themethod according to claim 6, further comprising the step ofcrystallizing at least a portion of the oxide semiconductor film byrapid thermal annealing using light.